His age or his height really have nothing to do with the correct bicycle size. It is based on inseam. Take him to a store where he can sit on the bikes to get an accurate measurement. Wal-Mart and Toys-R-Us both keep several bikes on display. 16" is probably a good starting point. He should be able to sit on the bike's seat with both feet touching the floor, if not the bike is too high.
P.S. To the first guest...lighten up...she wasn't sure how to get the right size so she posted here, isn't that what this site is here for?