if he's being homeschooled I probably wouldn't suggest any mature rated games
I mean, there are always exceptions, so I'd just call and ask him if that would be ok or something. but if for some reason you can't ask, I'd stay on the safe side, and avoid M games
forza is always a safe bet. it doesn't have anything that a parent could really disagree with, and it's fun for most people. forza horizon is a little more questionable, because there's some illegal street racing in it, but it's most likely to be enjoyed by any regular person, while with forza 4 or something, someone would need to be more into cars or racing
but really I'd just ask what he wants. you don't want to get him a game he already has, or wouldn't like