Get him on a small 26 and a 24 in bike. let him ride it and try it out. Last summer, my son was on a 16. He was too big for it. Got an old 20 in giant from a friend, thought it was perfect. discovered after two weeks my son wasn't riding it and was taking his little brothers 12 in bike. the pedals and seat fit him perfect. His arm were fully outstretched! he couldn't turn properly so he wouldn't ride it. Got him an 18 Royal Baby off FB and he road it all summer! Just got him a Gary fisher 20in with gears that he is tearing it up on and I can't wait til the 4th to go to my old MTB place and give him his first taste of single tracks!
If the bike is too big for him, he won't ride it and he could loose all interest in riding.