Coaster Brake for kids
One of the advantages of a coaster brake is young children typically have stronger legs than hands. They can apply more brake pressure with the legs than the hands. Many children however have difficulty squeezing the lever with enough force to stop the bike confidently. In my experience the lower the bike quality the more effort required to actuate the hand brake. Some of this can be alleviated by proper adjustment of the lever movement by use of the lever stop screw. This adjustment brings the lever closer to the grip so smaller hands can get a stronger pull on the lever. Many entry level brake levers lack this feature. The disadvantage to the coaster brake is it requires that the crank be rotated backwards to actuate the brake. Most people have a dominant leg similar to be left or right handed. In a panic situation the pedal may be out of braking position delaying braking. Another disadvantage of coater brakes is they are totally ineffective if the chain falls off or breaks. I would teach your son to ride using the hand brakes. Practice stopping drills on a level surface. Perhaps draw a chalk in the drive-way and see how close he can come to line without going over it. With my children as we rode along the local rail-trail I would occasionally tell them to "Stop Now" so they would gain confidence in stopping quickly.