I think there is an easy answer. Negate the holdback/prefirst parents who are attempting to gain an advantage for their kids (not all, but plenty are doing it for that very reason) by signing up for your club's team that best fits your son's "physical" age. Birthday doesn't matter, nor does school year. Remember, it's not a "grade based" system, but rather a "graduation year" system. All you have to do is be willing to consider re-classing/PG/holdback in the future to meet the requirement of picking your son's graduation. Then the parents who want physical parity will get it and the holdbacks will lose their advantage, and there's nothing they can do to stop it.