You should definately add a front brake to the bike. Although there are already two brakes (meaning you can still stop the bike if one fails) the front brake can stop the bike more effectively than the rear brake and it's a good idea for your son to get in the habit of using it rather than relying on the less effective rear brake. The coaster brake or rear handbrake are ok for general slowing down, but knowing how to properly use the front brake may one day save his life out on the road.

Caliper brakes require different levers to v-brakes. Make sure you get the correct lever (and don't let anyone tell you there's no difference) as using the wrong lever type will result in a brake that feels nice and firm, but can't be pulled hard enough to be any use.