Unfortunately, being an idiot is not illegal most of the time.
If this person were to discriminate against you in very specific situations like housing or employment based on his views, then it would be illegal. But there is nothing that says he can't make ignorant comments in other situations, like the one you were in.
It sounds like you are doing about all you can do by letting the owner and/or general manager of the dealership know about it. You might also want to write a letter or email to the customer service department of the manufacturers the dealership carries (Honda, Ford, etc.).
Sorry you had to face this. DD13 is Chinese (adopted) and she is starting to have to deal with a lot more racial comments and stereotyping. The racial "jokes" other kids tell hurt her the most right now. It sucks.
Yeah, my kids were also adopted from China. It is amazing the stuff adults will say. People seem to feel free to say idiotic things about Asians that they would never say about other minorities.
My kids generally call other kids on their stupid comments.