Although Abby said that perhaps people perceive that he is a little off, and that's why they don't respond, I thought it was probably the reverse. People on the street don't perceive the disability. Having a stranger walk up and greet you on the street for no reason is a little intimidating. For example, it can be the opening for an obnoxious sales pitch.
Years ago, I worked with the chronically mentall ill, and I had a client who did this, but he was so obviously disabled, that most times, people would say hi back.