Please tell me you're going to clue him in. If he's oblivious to this stuff, he wants to go to the party but he doesn't know why, somebody's got to tell him.
I don't know if he can solve his hygiene problems. It can be pretty tricky to do, and some of us really do need help our whole lives with that kind of thing. But--well, he deserves to know. Be very careful to tell him that you are telling him because he deserves to know--not because you want to make him feel bad. Sometimes telling somebody they smell is considered an automatic insult, and some of us have been bullied pretty badly in the past and don't know when it's not an insult. So you'll have to say straight up that you do not intend to insult him.
That said--there are more things in life than parties. Maybe he just wants to go because he figures people his age are supposed to want that. Maybe there are things in life that he likes better, but isn't open about talking about because it's considered odd to like them. Are you guys still in high school? If so, that can be a really high-pressure environment.
But it sounds a bit like he's kinda doing some stuff just to get a reaction out of people--like, he hasn't got a clue how to interact, so he just does what he knows will get people to react to him, even if they react with disgust. Or maybe he just doesn't think fast enough, and does things impulsively. I think he's probably going to have to do a lot of learning. But don't give up on him as your friend--even if he's horrible at socializing, it seems like the two of you get along okay, and it can really help to have somebody around who doesn't treat you like an outcast. People do learn as they get older, if they keep trying; and having a sensible person around may be just what he needs as incentive to keep learning. Maybe in five years, he'll be where you are now; that's not unlikely. You should've seen me at fourteen... good gracious, I was horrible! But I've gotten lots better, learned a lot of useful stuff, and smell a lot better; and I have faith that so can he.