As others have said, adding a front brake is a good idea.

You said "Probably one more year on it before we have to upgrade him to a larger bike." Don't make the mistake that many parents make of thinking that a kid should be put on a larger bike as soon as possible so that he can "grow into it." You could probably keep him on that bike longer than you think. Most kids are perfectly happy to stick with a 20" bike until they start driving.

But if you do decide to get a new, bigger bike for him sometime in the future, at least wait until he's really outgrown his present bike. Buying a dangerously oversized bike is false economy. You'll end up buying exactly the same number of bikes either way, so what's the point of putting him on a bike that he might not be able to handle safely?