Hi,
since you asked I'd like to add a couple of things to all your comments. I know that for the sake of your question you had to put "our black son", but I think that throws people off. He is your son! period, it doesn't matter if he is black, white, purple, green, yellow etc.!!! How do you do what's best for him? the same way you do what's best for your girls, you teach them all respect, love, self esteem, self-respect, confidence, integrity, the fear of God, etc. To him he is just your son! Yes time will come when he will ask how come he is different than the rest of the family, but what is more important is that he never feels less than the family, that although he is of different skin color, he still feels like part of the family; that he KNOWS he is your son and therefore a member of the family. Remind you, there are children that don't even feel like part of the family because of the way they are being raised by their parents. So it doesn't matter what color you are, or what color he is, as long as you all love and respect each other without making differences amongst you he will grow into a find young man. He will meet people his color and he will interact and play with other kids when the time comes. But don't focus on taking him to "all black" places just because he is black. When he goes to daycare, pre-k, kinder, etc., that's when I would consider a school with diversity.
I do agree that if you are happy at your church, don't change it because of that. But if you are not happy then yes, go and find a church that will serve your needs better, but don't focus in it being a mix church. If it happens to be mix, then that's good and if it doesn't it's still good as long as you get the teaching and support you are looking for. Please don't stress about this, just treat him the same and let him be himself.
Best wishes for you and your family. And remember, what's inside us weights more than skin color!!! Be blessed.