i would leave 36 on the front, look at putting 14 or 15 on the back that should work out better for him. work gearing ratio like this 36 diveded by 13 =2.76 x 20=55.3 so 36x14 =51.4 and 36x15=48.0. look at some bmx race websites you should find gearing chart i think theres one on "ultrabmx" or try "crupi". maybe look at fitting smaller bars junior or expert, bike would feel lot better to him...