Man calls me retarded?

I've taken the bus before and got on at the very front where the disabled seats are. I always sit there, but of course I make sure there's space for disabled people too, and whenever someone comes along, I always get up and move. Then a man came along who wasn't so good at walking, and I immediately made room for him.

Then he started giving me a rude dig, saying the seat was for people with disabilities. He was right, but I had made room. Before I could say anything, he said I was disabled too. I hit back with, "Are you serious?"

And then he said, logically you are disabled because of your weight.

I find his behavior a disgrace. 1. He has no right to address me informally, and 2. It's disrespectful to insult someone like that.

How do you see this?