I'd recommend a BMX, more bike for the buck, I'm 5'6" and I ride one. A lot of High School kids ride them to school.

Advantages:
everythings bolted on, requires one lock to lock up
bulletproof durability
minimal maintenence
roll smoothly, good sidewalk bikes

Disadvantages
pedal standing up mostly
no suspension

I have both and the 20" wheel translates into a lower gear for hill climbing. At age 10 he shouldn't need a mountain bike for at least 6 years. If he has to have one I think Gary Fishers start at about $159 with walmart quality components but assembled correctly.