I really have no idea how to handle it. I really don't want to lose his friendship, even if it means that it can't become something more. Obviously your little friend has some issues of his own to work out. I would hazard a guess at bisexuality or homosexuality that he can't bring himself to admit. Sometimes, nay, most times, even in hetero situations - when best friends have sex, things get weird.
Sex throws a big hard bone into the dogpen of friendship.
The only way you are ever going to resolve this is to find a way to get your friend to sit down and talk, just the two of you. If neither of you acknowledges that this happened, it is going to eat away at you until there is no relationship left at all. Be gentle and kind when you speak with him, and give him space to have whatever feelings he needs. Be understanding, and hopefully you can work out a resolution. I'm an overweight but fairly handsome guy.