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!