For this question there are probably two key things to keep in mind, 1) the internet, and 2) more options will appear on successive repeats.
Also note that a lot of this is based on the assumptions made by OP in their other (linked) question
At the end of his first repeat he can carry back something around the size of a thumbdrive, by his third lifetime he could carry a laptop, the size of things he can store will continue to grow linearly with lifetimes (possibly slowing down after he reaches a sufficiently large carrying capacity)
Without this assumption it would be a lot harder to take information back with them. Though the idea of behaving as a gifted child and interacting with other gifted children (see below) would still work without the significant stress of playing a child prodigy.
The hardest repeat will probably be the first one, since the protagonist has made no preparations for this and so has to go off spotty recollections of things that happened at that time. Since he is a software programmer he will be able to prove to his parents that he is from the future by March 31, 1999, the day the matrix came out. All he has to do is explain the plot of the matrix (which he can probably remember) before it is filmed. Alternatively he could convince his parents to watch monty python's Holy grail or life of Brian and quote half the movie from memory (Monty python tends to be well known in science and programming circles, and its not unreasonable to assume that he could do that even as an average person). Since his parents haven't shown him these movies its going to make them think (though their first suspicion may be that he saw it at school or at a friend's place).
There is also the internet, this would give him access to other things to learn, as well as the dot com bubble (which occurred between 1995-1999) which he'd likely be aware of, and might be able to recall a particular business that had a short term spike in share prices and be able to build a nest egg from that for future investments (This would be quite believable if it had a funny name or offered a ridiculous service that would never work in hindsight.) One example of where he could look is the arXiv, a preprint server for scientific papers, which (at least now) is accessible by anyone to read research papers on before they are published in a journal and I believe was connected to the world wide web in 1993. It was originally for high energy physics (which he'd need to teach himself the material) but later on included math and computer science papers.
From what you've said in the first loop he plays the child prodigy and that may go to his head. As far as he will experience he is a prodigy, he is much smarter than everyone around him and he is a similar age or possibly a couple of years older than his parents. This could have a significant influence on him and make him appreciate learning more in future loops than would be expected by his original life. He may also try to tackle big problems such as climate change because of his now inflated ego, joining activist groups and possibly trying to integrate himself with the scientific community. Somewhere between the ages of 15 and 20 is probably when the truth that he isn't a genius and he fades into obscurity (with maybe only a wikipedia article as his legacy, not too unusual for child prodigies). An average 40 year old may be comparable to a genius 10 year old (though a psychologist trained in high intelligence children might see a confusing difference in behaviour), but an average 50 year old will be struggling to keep up with a genius 20 year old.
In his later life he'd be able to decide to take a USB back with him, given he's probably invested in google, amazon, walmart, youtube, facebook, bitcoin, ect he is independently wealthy and doesn't have to work. He can also afford to get someone to design a USB that is durable enough to work after decades and can interface with multiple standards (so he can use USB from 1996, and something else prior to that). Now he'll be able to send a Terrabyte of information back in time (though some of it may be blocked off until later dates due to hardware limitations). While he is now again only of average skill, he does have more training and experience in more developed/efficient programming techniques then most of his colleges, such as agile software development or test driven design.
NEXT FEW REPEATS
The next few lives is where things get interesting. While he doesn't want to be a child prodigy he can pretend to be gifted and get to meet/befriend actual high intelligence children. These "childhood" friends would be much more interesting since they would be able to introduce new ideas/concepts much easier then the average students (and gifted children often get along better with adults then similar aged peers). Since they are actually gifted they may manage to guess that he is from the future (particularly if they like science fiction). But now the fact he is only of average intelligence will no longer limit him since he will be able to get help from his good friends. He will probably still spend time with kids of average intelligence since they are actually closer to him and he won't be bombarded by insightful links between ideas that require him to think hard to follow, though there will probably be a barrier between the average friends and his gifted ones.
He will also have access to programming languages and libraries that don't exist for a couple more decades. These programming libraries are very useful as they don't require that a user understands how to program everything from the ground up and can be made quite efficient. This means the protagonist could take google's tensorflow and use it to start a different search engine company that out-competes google due to using their future (superior) technology. However there will be some limitations since the hardware back in the 90s wouldn't be able to run these algorithms very efficiently and so he'd have to wait until technology gets better.
He could also start chatting with researchers anonymously via email, claim he is an independently wealthy researcher who works on things in his free time but not linked to a university (and buys access to journals which he has access to from his hammerspace USB). This may be motivated by his (failed) attempt to solve climate change as a child prodigy. If you've seen the limitless series think about the communication between Brian and the guy who extends a mouse's lifespan.
Since intelligence is no longer an issue by the third time through the USB can probably be designed so that he has access to everything immediately as well as some instructions he could use to boot strap better tech from what he would reasonably have available given his age and the time, as well as better ideas of how to balance boring school lessons with necessary social interaction.
Later on once his hammer space has expanded he'd be able to send back something like a raspberry pi or a NUC (next unit computing by intel, you can buy one now with 16GB ram, i7 core/4GHz and 1TB of storage). This would allow him to make use of the software he's been carrying back with him, but would probably need to be re-engineered for robustness and to interface with monitors/keyboards/mice of the 1990s. How long this takes may depend on how quickly his hammerspace grows
Eventually one of his gifted "childhood" friends could invent a strong AI, this means he could finally take someone back with him and keep himself entertained with this character. Later depending on how you want to take the story things like intelligence augmentation or neural implants could be developed. This means that when starting at 5 years old again, instructions may be included on his original USB (and possibly guidance by his AI friend) on how to boost his intelligence or implant a neural connection given only his tech level/intelligence (since he will suddenly become dumber when reverting to 5 if he manages to augment his intelligence). Once he has a neural implant he can chat to the AI or write software/read research papers during the long division lessons. If brain uploading is also developed then he could bring people back with him and essentially be a hivemind spending time talking with himself during the classes for things he has long since mastered.
If he decides to focus on this it might be possible to develop after several cycles (and either he or one of his gifted friends could come up with the idea). This is especially true once the strong AI can help by bringing an understanding of many advanced manufacturing techniques, biological processes, and software approaches back with the protagonist.