It didn't take long to find my son a real mountain bike. I got him a Trek with 20 inch wheels and a grip shift controlled six speed rear cassette. The bike is adjustable as he grows with a customizable stem, and two different sets of holes on the cranks. We broke the bike in today in Russian Gulch State Park near Fort Bragg, CA. My son borrowed my Pearl Izumi arm warmers for his legs!
More pictures HERE.