Bob,
What is your connection with the team? Are you a coach or just a parent? Also, what level are you talking about? How you approach this fairly common situation may depend on your relationship to the team and the level of play. Express your concern directly with the assistant coach if you must. If your concerns are ignored, then take them to the head coach. If you still are not satisfied you have two choices-accept the situation, or quit.
Please allow me to share a similar situation that I had with a coach of my sons baseball teams. This coach was very good at teaching skills, motivating, and caring for his players. He had the whole package and the parents and kids liked him a lot. But he was so obviously favoring his son and promoting him over everyone else that we, as parents, would chuckle in disbelief. After a while, the son figured out that he didn't have to produce like everyone else-he would still be starting and batting clean-up no matter how little effort or poor his attitude. Eventually the charade caught up to the coach when his son didn't make the high school team.
You need to decide what is in the best interest of your son. Is he learning good skills and having a good time? If he is, bemuse yourself with the antics of the coach. The coach won't change. He too is thinking that he is doing what is best for his own kid.
My advice? Enjoy watching and supporting your kid from the bleachers, do not badmouth the coach in front of other parents, and hopefully this will all be over at the end of the season.

Bob,
Let me get this straight; One head coach, two official assistant coaches, and two unofficial parent helpers? Sounds to me like your head coach needs to take control of this situation. There is only one head coach, giving one set of directions, that must be understood and followed by all coaches and players. One leader going in the wrong direction is better than two leaders going in opposite directions. If the head coach is unwilling or unable to take this responsibility, he must be replaced by someone who can. Who has been given the responsibility to watch over these parent volunteers? You are right, coach in-fighting and disagreeing in front of players and/or parents is unacceptable, and must not be tolerated.
Make your head coach aware of the problem.

Bob,
Go for it. It is very unusual to find a coach who doesn't favor his own son. Get him if you can find him. Otherwise you're left with two choices; pick a coach without a son on the team, or pick a coach with an outstanding athlete with a great attitude as his son.
Best of luck to you.

Jack,

You noted the key item ... the Head Coach needs to exercise control. Talking with him offline may be your best (and first) step. With some of the issues we have faced over the last four years the best person to bring it up is the one that doesn't appear to have an ulterior motive (i.e. the parent of the kid who was displaced is NOT the best to address it, but one of the coaches should be in a better place).

Good luck