A hand brake bike may be hard for him to grasp unless he has some big hands. My 6 year old has a hard time in his 20" mtb, not so much on the 16" coaster combo. I would say get a 16" bike with a coaster and front brake. Get him using the front lever (I had to adjust my sons so the lever was almost all the way in at first and adjusted the brakes as he grew), as that's the one you should use to stop any hand brake bike. The coaster will be there as a backup and if he is feeling confident then take off the rod and install a rear cantilever brake also.