If you're invited to the things but just feel awkward and not able to mesh well once you get there, that's different than if your friend stopped hanging out with you in lieu of new friends.
It's hard not to assume the worst, I get it, but you'll have to give the benefit of the doubt and assume that your friend does have new friends but still values your relationship the same... invite him to things or try to make the plans you usually would, and if he says he's busy then try to figure out a time that works, but avoid getting flustered or agitated if possible. I think it will become apparent if he is avoiding you or wants to "move on", or if he is just getting busier with other things and new relationships. Maybe you can't hang out as much as you used to, but that doesn't mean your relationship is no good.