S.T.
if he's 'insisting' it sounds as if you're pushing. i get it- it's very hard to perceive that our kids are dealing with social issues. but you don't really know what's going on here because he's telling you everything is fine.
maybe he's telling the truth.
or maybe you need to back off and use a lighter touch.
driving in the car can be such a great place to have deep conversations. something about the enforced lack of eye contact, perhaps. you might think about opening it up by sharing something about you, maybe from your teen years. this can encourage him to open up without him feeling as if you're trying to pry at him.
you can't fix his friend group. if they're constantly excluding him, and he's not doing anything to create animosity or distance between them, they're not really his friends. you can't shoehorn him back in, nor can you create a new group for him.
you can help him with some introspection, some decisions as to whether it's time to find new friends, or to work on himself, or to do some personal shielding, or to get involved in a new activity, or even, yes, to take some time away from seeking social contact and enjoy the benefits of solitude (but do watch out for depression.)
there's not really enough here to make any useful conclusions, but you won't find out what's going on by trying to pressure him into telling you.
teenagers can be a tough nut to crack. good luck.
khairete
S.