Pick up an RC-10 RTR. I was into RC off road racing when I was younger, and had my share of Tamica and then Kyosho (Ultima Carbon) buggies. I got back into it a few years ago with my son. I didn't have time to buld, so I grabbed two Team Associated RC-10 ready to run set ups and a couple of extra Li-Pos. It's a blast, they're fast enough to really enjoy in the back yard, at a soccer field, etc. My son digs it. We fix them together after our crashes, etc. There's also a ton of upgrades, so if your son starts getting into it for real, he can swap out just about everything on the car with better parts.
Tower Hobbies is a great place to start your search. Might not be the cheapest, but always has a wide selection
RC10 rtr:
http://www3.towerhobbies.com/cgi-bin...=LXFLVBBY&P=SM
Here's the whole list:
http://www3.towerhobbies.com/cgi-bin/WTI0297p?C=CA