How do I get my 4-month-old puppy to stop licking his penis?

I have an American Hairless Terrier. The first time he humped someone was on February 7. Ever since then, he has been acting like he is obsessed with his penis.

Whenever I hold him (I am his primary relationship), he licks his penis. Twice this week he licked to the point of having a full-blown errection. I stopped holding him when I realized what he was doing.

He has not performed this obsessive-like behavior in front of his other mommy, although he will have an occasional lick. He does respond to “No” and “Stop,” but only momentarily.

Note: He is scheduled to get neutered in two weeks.