He lashes out at you and treats people at the party poorly as a defense mechanism because he feels inferior to you and others at the party. He subconsciously or not wants what he sees you all getting: validation from your peers. It’s a basic human need. This is probably why agrees to comes to these parties with you even though he’s afraid, he wants to build friendships and he wants to feel connection to people but he’s too fearful to be himself and that anxiety takes over.

Baby steps are key, maybe next time there’s a party drive there without him and tell your friends he’s busy but might drop by so he can decide if/when to come and leave when it gets to be too much for him. Participate more often in the activities he does feel more comfortable with so he can have more positive social experiences, build confidence, and feel more comfortable with your friend group. Too many negative social experiences and too little positive ones will just validate his fears and he’ll never overcome this behavior.