The answer depends largely on the setting. The country & year for Earth... and in any case, the details of the culture and laws and who is looking for him and how they do that and what he and whoever might help him know about the search methods of the people looking for him.
I think it's also vital to look critically at what "normal" means to this person, to different groups of people in the culture where he is, and to you as the author/worldbuilder. There is of course no one true "normal", and the situation you describe is set up to explore that, and the blindspots the author and readers and the culture have around what "normal" means.
As for your practical sub-questions:
How could he become a citizen?
Depends on the culture, laws, administrative details, and the person's knowledge of those, skills, and the situation with the people looking for him and his travel opportunities. Before the 21st Century, it was much easier to do even in Western cultures. If the person has infiltration training (e.g. if he was trained to be an anonymous agent something like Jason Bourne), he may have been given many skills and techniques for doing this sort of thing. It may be helpful to travel to a different country before trying, e.g. by sneaking aboard a freighter, or crossing a border via deep forest, or stealing a private boat or small plane.
How could he attend college without having ever gone through high school? He has already been taught roughly up to a middle school level.
There are free online college courses available that don't require anything other than an email address, or perhaps less than that. Or there are foreign schools. Or he can study the admin offices of less rigorous colleges and figure out how to fool them. Some admin offices aren't terribly foolproof and could be fooled. If you look at stories of people struggling to correct their information with admin offices, and turn them around to realize that some students have been going to school with wrong information just by clerical accident, then you could see how it would be possible. But the person needs to somehow realize this is a possibility, perhaps by meeting someone with experience who gives him the idea. If he's has some sort of human manipulation skills, too he could just get the support of someone in the admin staff (e.g. by seducing them or bribing them or blackmailing them or intimidating them) so that they fake his credentials for him. Or maybe he simply has (or knows someone who has) forging skills.
Note that he could also do things like join the French Foreign Legion, or a religious order or other group that is willing to take in humans and vouch for them as a member of their group, and that has a trust relationship with governments.