Hi, it sounds like all that you really need to do, at least initially, is find out why he seems to be more distant. Just ask him. Not in an accusitive tone or anything like that. Just be upfront and say something like "Hey, I feel like you've been becoming more distant, what's going on?" See what he says and then from there you see in his response whether his tone is congruent with what he says. If you sense incongruency, then there is a bigger issue at hand. At which point I'd recommend being a little more direct because you don't want to waste time. If he is not strong enough to respond to you clearly, which you'll be able to sense again because of the incongruency in his tone and words, then just drop him and don't waste anymore time.
I don't recommend seeing other people until you've done the above. That way if he clears up, you don't have a mess to clean up. Hope this was helpful.