Helping Goalkeeper Mental Strength for a 10 year old?

Hey guys.. how do you keep up your mental strength when you have rough games? My son has been down on himself for the first time since becoming a GK. He is a lot better now but it was a rough 3-4 weeks. His team had been on a win streak for a while and all of a sudden they lost 4 games in a row and they started saying some mean things to him (and im sure to each other) during the games. Usual 10 year old stuff such as "you could have gotten that" "you suck" etc. He's never faced this before. He all of a sudden didn't want to play anymore or be GK. I spoke to his coach and he spoke to the team to make sure they remember they need to build each other up and that it has to go through all of them before it gets to the GK. Anyways.. I think he's better now but how do I help him in the future? What gets you guys motivated? The coach said that hearing negative comments usually motivated him to be better but I obviously don't think that works for my son. How do you keep your mental game up and ignore mean comments? Is it normal for teammates to say things like that?