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.
Goalkeeper (association football)
The goalkeeper, often shortened to keeper or goalie, is one of the major positions of association football. It is the most specialised position in the sport. The goalkeeper's primary role is to prevent the opposing team from scoring (moving the ball over the defended goal-line within the frame of the goal). This is accomplished by the goalkeeper moving into the path of the ball and either catching it or directing it away from the vicinity of the goal line. Within the penalty area
Cassandra Dimovski
Cassandra Dimovski (born 27 December 1993) is an Australian football (soccer) goalkeeper who most recently played for the Melbourne Victory in the W-League, the top division of women's soccer in Australia.
Early life.
Dimovski was raised in Lower Plenty, a suburb of Melbourne, Victoria where she attended Templestowe College. At age 14, she was scouted by a teacher to play for club team Eltham Redbacks. Originally playing as a midfielder, she changed to the goalkeeper position. Dimovski also