I have played in goal for various school and local teams for 23 years now and have faced my fair share of abuse from well, everyone.
Your son should realise that the important thing is to enjoy the game.
If he makes a mistake or has a poor game then he should be encouraged to look at why he missed the pass, or conceded the goal or missed the shot. Then he should resolve to practice and better himself.
Tell him that even premiership players have bad games. Look at the lads last night!! Tell him that those players will all be feeling really bad but they will be back with their regular teams on Saturday and will be trying harder and getting better.
If at first you don't succeed, try, try again.
Good luck,
K