Yup, Beerman got it.

You don't want to use his friends bike for comparison. His bike almost DEFINITELY is running 39/14 gearing. Which is very close to 36/13 gearing. But, you may want your son to be able to pick up speed a little quicker, even if he can't go as fast when driving as hard as he can. It'll still be plenty of speed and help him develop his muscles if he goes with 35 or 34 on the front sprocket.

You may have to pull a link out of the chain - get a chain breaker for 5-10 bucks if you don't already own one.