I think the tactics you are trying are the right ones. I have seen numerous cases in which a player who plays on a team coached by one of his parents will push the limits of not paying attention to Mom or Dad but will snap into line when instructed or confronted by another adult. So, having your son talk to other adults and athletes he respects is a step in the right direction. I might take that one step further and have a coach he might want to play for down the road - like the junior varsity or varsity basketball coach at the high school he will attend - speak to him about the importance of character and what he looks for in a young player.
While it never is a good idea as a coach to punish your child more severely than you would the other kids, perhaps creating a team policy of taking kids out of the game who complain to refs or taunt opponents could send a message. Another potential avenue is to maximize the time you spend with your son attending sporting events or watching games on television. Point out sportsmanlike play and players who display great character and contrast that behavior with inappropriate, unsportsmanlike actions and ask him directly which players seem more likable and which behaviors are more appropriate.
