We had this happen!!! I was so humiliated!
We moved from a dominantly one raced area to a more mixed race area and when Keeley first saw that there was other races it suprised him and he barked.
I started taking him to parks to "people watch" he just had to see that it isn't a big deal. I went up to one family and started to talk to them and shook the dads hand with him watching, so he could see that it wasn't a big deal, and he has chilled right out.
I have read about this happening. Not only with races, but gender, weight/size and ages too. If a dog gets used to only one type of person they get uneasy around what they haven't experienced. My puppy training teacher said to try to get your dogs to meet and have positive experiences with all people.
I think if you can have the new family pet him, maybe give him treats and let him see that there is nothing different about these people he'll be just fine.