If you really want to monitor his usage, deleting the market isn't the best way to go about it. I'm 20, by the way, so... I have a more mature brain but I'm still in touch with my adolescent side .
The best thing for you to do would be to tell him that he can have it on the condition that you get to periodically look through it to monitor what he's doing. But don't tell him when you're gonna do it. Just randomly take it and look through the browser history and app drawer. (That way, if you randomly take it from him, he won't be able to clear the browser history before you look). Also, you could look through the MMSes if you are concerned that he could be sexting or something, LOL.
You can unlock code protect a Droid, but just tell him as a condition of you getting one for him, he has to tell you what the code is (so that you can look through it, obviously).
You have to have a credit card to purchase apps from the Market — they don't get billed to your phone bill, so you don't necessarily have to worry about that, haha!
I think, while it's a little harsh, it's pretty reasonable that if he wants you to pay the bills for his phone, then you get to see what your money is going toward. He may not like that setup too much, though, haha! :P