Maybe you should tell him the thing about the fat chick line, and how you'd feel the way he did -- so you understand why he's upset. But point out to him that you two were talking about his flaccid penis, which has nothing at all to do with how big he is erect, and the words just came out of your mouth since he brought up "small", that all you were trying to do was make a cutesy comment that related to what he said. Reiterate again that you understand why he feels that way, that you didn't mean it and you're sorry for hurting him and then blow him silly and keep blowing him until he feels better.