I would like to think that this boy is just picking something to pick on him about iykwim, not that he is a racist, but then who knows what he hears at home?
He is obviously a bully who is being nasty to your son and that needs to be addressed.
I would struggle to brand him a racist on this incident alone, he needs speaking to to get to the bottom of his obvious dislike of your son and to see where it is rooted.