I'm with COG on this one. Look, if he is mature enough, and driving is his living - he'll behave himself. I've got a good friend that is in the same profession and he doesn't mess around when it comes to that - knowing he could lose his job.
But also, I think a joking "behave yourself" (what my W and I say to each other) or like COG says, wish him a good time with a call me if you need a ride should suffice...
Sven
Never sacrifice the great for the good. Sometimes the beauty of grace is that it makes life not fair.