I got my son a Redline BMX 16" wheels that is now discontinued but looks a lot like this:
Had to special order it, but worth the wait. He loves his bike, it is way cool. However, a rear coaster brake may be necessary if your son's hands aren't strong enough for hand brakes.
P.S. Oh, wait, your son is 3 y/o? Then this bike won't work, too big.