also gear size depends on the diam of the wheels if he is on 1 1/8″ or 1″ that makes a big diference rather than if he is running 1 3/8 which is on many jr bikes that come stock.
check your gear chart and let him run an easy gear that gives you about 54 give or take a few…. let him spin if he can i wouldnt start with big gear ratios esp if he is young he prob dosnt have great leg strength.