Well, if he considered you a friend, then you might want to have some words rehearsed if he follows you and corners you and demands to know what's going on. Something along the lines of, "Dude, I'm not ignoring you because I'm mad, I just think the friendship deal between the two of us has run its course. I wish you the best--really--but I'd rather not hang out or talk anymore. Take care of yourself." Simple, direct, respectful, but not leaving room for discussion.