There was this kid I coached once who had tons of talent but was weak mentally and liked to complain that the coaches didn't like him because they were always getting onto him. My buddy I coached with and myself were talking to him one day and my buddy said; "don't get upset when coaches get onto you because that shows we care and trying to get the most out of you. But you need to worry when we stop getting onto you because that means we've given up on you and won't try to get the best out of you."
I thought that was some great advice and goes to show that just because a coach is tough on you it's not the end of the world. There are tougher things in life than getting yelled at while in a sport.
Tell your son everyone goes through this and it's not anything unique to him. He can either grind through it and accomplish something amazing or he can be just like thousands every year who just aren't tough enough to make it and quit. Then it gets easier to quit from there on out.