DO NOT ALLOW ANYONE TO BULLY YOUR CHILD BE IT A COACH, ANOTHER PARENT OR A PLAYER AND DON'T LET YOUR CHILD BULLY ANYONE.
Would you stand by any at the playground or at your house if someone bullied your child? Would you just stare and say nothing? I would hope not. It’s no different in soccer. NO ONE has the right to bully your child. Conversely, your child does not have the right to Bully anyone.
You are the parent and you need to stick up for your child and give him the tools to deal with bullies. If the bully is a parent or a coach, you need to stand up for your child. Children are taught to respect adults and there is no way for your child to stand up to a bullying adult, but you can and you should. If someone makes a derogatory remark about your child within your earshot tell the person to stop. Its as simple as saying, "Feel free to speak about your child anyway you want. But DO NOT say another word about my child." Most times, the person, when confronted, is taken aback. If for any reason, this does not work, then quietly and respectfully tell them to stop. 99% of the time this is the end of the problem. If you are dealing with the 1%, let your coach know and speak with him about how he will stop this from happening in the future.
Bullies are everywhere. You can be a role model for your child in how to deal with bullies. Don't expect your child to "take-on" an authority figure. That being said, they should not "take-on" an authority figure by being disrespectful. Example: A player goes to a ref and makes inappropriate remarks or finger gestures. At that point, your child is a bully and you should remove them from the field. If your child is being disrespectful, pull them off the field. If that's not an option, and it always is by the way, talk to the coach and ask him to bench your child. Bullies are created by those who don't stand up to them and by those who encourage that type of behavior.
Whatever you decision, do it with respect and model the right behavior for your child.