If he is really into racing games get him a decent computer and rfactor. Otherwise the 2002 world of outlaw game or a game called Saturday night speedway (both for ps2 only I believe) are the only dirt games I'd recommend. The other ones are just so arcade-like that it is a disgrace to dirt racing.
Guys, his son will want to play online because it is not 1992 anymore. Foytfan, the only game available for Xbox is WoO. Saturday night speedway is very fun but after 2 hours of playing it you can lap the field, and it isn't an online game. He may enjoy Forza Motorsport.