It depends what he's interested in, and the time frame he has.
As stated in many of the answers here, he might not be able to find out exactly everything that happened since he left in minute detail. Historical documents may be lost/destroyed and he cannot find out what happened immediately after he left.
Also, he is unlikely to look at everything that happened right after his departure even if it was available. If you think of the amount of information available today, you couldn't possibly learn every sporting result that happens around the world without spending the entire day doing it, never mind learning stuff that happened 100 years prior.
However, it entirely depends on what he is interested in. If he is a businessman before he gets sent forward, he would likely find out what businesses made it big since he was sent forward in time. If we was a sports fan, he would want to find out who won each superbowl/world series/world cup (depending on his specific sport of interest) since he left, just out of curiosity. If he was into politics, he'd find out all the presidents since he left. That wouldn't take much time to do.
Personally, I'd want to find out things like which books are now considered classics in the year 2100, that were actually written since I left. I would also maybe want to find out what video games had been made and how they developed over time from when I left until 2100.
These things will likely be common knowledge, and wouldn't need to be researched, you could probably find someone who knows, and in the year you were in the future you would definitely have found out.
Once he has been sent back, even if he doesn't remember every result or every outcome, he would have a general idea. He would almost certainly remember what happened in the 1 or 2 years after he left.
The important thing to do now is to prove he knows, but without telling anyone, because it might affect the future. So he gives someone a sealed letter, to open once the event (election, tournament etc.) is finished. If the information is correct, that person will know he is from the future.
Think about it. In a sporting event like the world cup you have 16 teams in the final tournament. If anyone picked the correct 2 teams in the final, and then which of those wins (even without a score), I'd be pretty willing to believe they'd been to the future. You would only need several other minor details to prove beyond a doubt.
So even if he only has 5 years, within 2 he could convince plenty of people. If he has 50, he can play the long game and successfully predict most, if not all the outcomes, and convince everyone.