Like others have said, don't worry too much about the position your kid is playing (although it sounds like your son is the one that bothered the most). Yes, at 9 the coach should have the players playing all positions, but its probably not going to impact his overall development. Sounds like he is getting quality training and touches in practice. Just FYI to all parents, kids don't get to pick their position, but, at least until they're in they're early teens, they should be getting time at various positions. It's rare that I see a kid get pigeon-holed in a particular position for the life of a kids youth career (unless a player chooses to play keeper).
A coach may, or may not, have a kid playing in the "correct" position, but chasing a specific position on another team is usually a bad idea. More important than the actual position is what the coach's philosophy is on positions. If the coach is rigid on the role of each position and doesn't allow some free movement (push up, drop back, switch, etc) that is far more harmful to a kid's development. It limits the decision making opportunities of a player.
All this being said, it can't hurt to tactfully talk to coach about you son getting some time at another position. The coach will either 1) not a give a shit, or 2) consider give your kid some time at other positions.