This is the equivalent of a boy dipping the cute girls pigtails in the ink well. He likes you, probably more than you like him and he doesn't have the skills to show this properly. I don't mean "likes you likes you", just in a friendly way. Expecting couth, appropriate behavior from males is not always reasonable. Many of us want to be friends with a lady, just friends, and don't have the skills to pull this off... so we tease. It's the interaction we are after. A little gentle ribbing back in his direction may really help him feel comfortable with you, and give the vibrato a break.
Suggestion:
Him: Hey nice day huh? How's the rat dog?
You, overly sweet and with genuine concern: Rat dog? Awww..... you rode the "short bus" that must have been awful for you.
He's now on defense and it's a little uncomfortable for him, not someplace he likes to be. He may not let it go that easy but short little returns of pity towards him following his insults will have an effect. These comments should be given with the same feigned concern that a loving mother would use, there is no defense for this from the male perspective, it's just time to change the subject. Have a little fun with it, you may wind up with a loyal friend.
