I suppose you have taken the coach aside and explained you concerns to him already?? Failling that...........
Im tempted to give you advice on sticking it out and telling you to be strong and stay there etc but we are discussing a child. A child who is becoming ostracised by his coaches racism. We all have to get used to racism of the world and Im sorry your young one has to see it so soon.
I would take him out of the team and find one more suited to him. As an adult you are confident and strong enough to fight racism and confront it but as a child all you see is that no one will play with you. I wouldnt use a oung one to make a racial point by making him stay there in a hostile atmosphere.... Its unfair and unncesary also I dont like the idea of you paying money to support this mans wages.
Take your money and child elsewhere and complainrelevant authorities about the coach. List all the things he has done or said and find an independant advisor and go to the sports board about him. You say the other parents have noticed the unfairness, they may or may not back you up. Im not an American so I dont know the way it works or the names of the organisations but someone here will be able to tell you. The important thing is to get your son away from there.
PS I edited out the names in your post as putting such things online can be dangerous whn added to your location