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.
I have to say, your spot on view made me remember a TV interview on parents intentionally "red shirting" their kids to be better at sports. My now 11 year old kid was a baby at the time of viewing the program. I thought it was insane that any parent would intentionally do it just for sports.........but the reality is some parents really need to live through their kids