Safty is the most important thing. If you can't fit "correctly" it's only a matter of time when he gets hurt. I have 3 boys that ride with me; 7 , 10, 13. My 13 year old has about outgrown is 13inch hardrock, so we though we might get my 10 year old onto the 26'' mtb. My 10 year old is kinda short, but he had a clearance over the top tube, Just not alot horizonaly (the bike tapers upward to the headtube quickly). Anyhow, he fell and broke his arm.
I wish I had not put him on the bigger bike. A broken arm cost me arround $1300 (with insurance), and the loss of my riding partner for a season. I'm just glad it wasn't worse.
Id'e spend whatever I could to make sure he was properly fitted.
You could also look for a set of trigger shifters to swap out for gripshift.
Last edited by Lets_roll; 04-06-09 at 08:36 AM.