I'm going to be brutally honest, and talking from experience as a youth coach myself, I feel I can give you a bit of insight. Let's get down to facts. A lot of parents and subsequently their kids are pussies. They cannot handle any form of aggression and this soft natured bullshit does not belong on a football pitch. Here in South Africa disciplined aggression on the field gets you far in the world of football. And unfortunately I have had to deal with soft parents complaining that precious little Jimmy got a knee graze from a good tackle. They need to harden the fuck up, because your kid and yourself are now doubting his discipline both on and off the pitch. You two are not the problem, the other parents and kids are. As long as it's controlled aggression, you have no need to intervene and the other parents need to accept that football is a man's game, if they do not like the way some players control their game with aggression, they need to find a new sport for their kids. Have a chat with him, and try to let him know that what he is doing is not wrong, however he needs to be as controlled as possible. Which is a weird paradox, but if he can control that aggression, he will go a lot further in both football and life.