He's always going to be behind his full potential until that heals up. The good news is that his full potential could be really high such that his pluses and minuses balance out. Some of our top athletes have issues that they overcome and their compensation skills prove to be an advantage.

Coordination / balance grows from the inside out, from the lower back then to the abdomen and up the spine. The only thing to do is more of whatever he likes to do. Lifting small free weights is just as good as anything else. He'll learn to balance himself even though he thinks he's building his arms or legs.