I started playing D&D with my son when he was eight. We started with the new 3.5 Basic Set, and after me running it once for him and his mom (mom is a good sport!), he wanted to try being the DM. Six years later, he's running his own campaign, and D&D is a permanent fixture in our lives.
I know another dad with boys closer to your son's age, and they like D&D. If you like, I can pass your contact info along. Trouble is, they were looking for a GM, too.