I have heard what you do is have your son do a few pushups and then hold is body up with his arms straight. Measure from picky to picky and that will be a good starting point. Buy a few different cheap sets of steel bars and experiment and see what he feels most comfortable with. You should have no problem selling off the slightly used bars at a track or local riding spot. Someone will eventually bend theirs up and need a pair.
If he rides tracks only, I would reccommend the stock width. If he rides tight trees a fair amount, than 29 to 30 inches total width is the norm. Any shorter than 29 inches may cause mounting problems with your levers.