When I was growing up, my parents didn't charge me rent as long as I was a full time student. Their arrangement was that they would pay for books and room and board for school (and I had to pay tuition). So, they figured it was much cheaper that I stay at home rather than in a dorm 30 minutes from home. When I quit school, I had to pay them rent, although it was very minimal.

I would definately charge rent, especially if he isn't going to school full time and has a decent job. After all, how is he ever going to learn to be responsible enough with his money to live on his own if you don't start to help him see how the 'real' world is where you can't just spend your money whenever you feel like it.