I don't respond to him anymore. If he wants to debate, he's got to do so in public.
There are some things that can be demonstrated by facts and can't be debated. The whole republicans are racist thing is very much played out, and the people who hold such beliefs are in need of new material. Hence why democrats will often resort to personal attacks and threats of lawsuits and even against one's life when presented with the truth that the democrat party is a viper's nest of bigots.