Every holdback has an advantage, you have to either get over it and move on or do the same with your son. I would chop my hand off before forcing my son to repeat a school year to give him an advantage on the lacrosse field. Deplorable IMO but many willing to do it.
You don’t even have to actually repeat the grade. Just say you’ll pg him later or do a gap year. Club owners will let him play down.
You can even take it one step further. Just have your kid tryout for a team at a grade below what he actually is. In all my years of having multiple kids playing at multiple organizations (and trying out at even more of them), I have never once been asked to verify their grade level or age. With other sports, that do it on age, you have to submit a birth certificate.