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.
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In most sports which involve scoring in a net, special rules apply to the goalkeeper that do not apply to other players. These rules are often instituted to protect the goalkeeper (being a target for dangerous or even violent actions). 
This is most apparent in sports such as ice hockey and lacrosse, where goalkeepers are required to wear special equipment like heavy pads and a face mask to protect their bodies from the impact of the playing object (eg. ball or puck.)
In some sports,
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