Like this:
You: Uh...so...you must see a lot of steroids and growth hormone and stuff.
Him: Sure.
You: Do you ever sell to people who aren't patients for sexual favors?
Him: Of course, who doesn't?
You: I'll lick ass for some GH.
Him: You bet your ass you will...
Hardcore porn music begins playing and the hilarity ensues.
Sorry bro, but I think you either flat out ask or you don't. Hope that he doesn't get upset. Your the one who knows his personality not us. You've got a better idea how he'll react.