..Hoping he can grow another 3-4" in the next year and a half, which would put him at 6'. ...
I know you commented on the reaching 6' partially in jest but be careful with that. I had that type of dialog with my son and it took him longer than it should have to accept his height and focus totally on what he can do with what he has instead of what he can't do because of what he doesn't have. If sub-6' is likely to stick, keep pointing out the players who have flourished who are in the same boat... guys who never, for one minute, let it stop them. It sounds like he is a battler and competitor. When things get really tough against the best of competition, having not even a seed of doubt will be a difference-maker for him.
Of course, if you or your wife are 6'4", never mind.