Have you played soccer or football? I'm a goalkeeper and the most important person on the team. 
I have never played soccer, i just put my son in soccer shots! Hopefully they teach him how to play the most specialized position
How old is he? Very cool. Push for Goalkeeper. Then he will be the hero of the team, or the 0 depending.
He is only 4, so he has a ton of learning to do to prevent the opposite team from moving the ball over the defended goal line!
As much as I love the sport I can't imagine how board you are going to be at all his practice games. You should join an adult league. 
I don't mind watching I wouldnt be very good at trying to catch or direct the soccer ball away from the vicinity of the goal line.
You never know until you try. Although I do think it's the most dangerous position. 
I could see how that's possible for the keeper or goalie to get hurt frequently since a ball is flying directly at them!
I've had my nose broken 4 times playing goalie. 
Oh my goodness, I can't imagine the pain of that! I can see why they have special rules to protect the goalkeepers