If you have the means, put him in a small private or religious school that emphasizes strict discipline. If you have the means to home school him, do this. The environment is the problem really. The public school environment is a bad one for a special needs kid. The public institutions don't have the resources to closely monitor kids for bullying. Unless you can get him out of there, you'll probably be playing damage control his entire childhood.
You could teach him to trade insults, or, if you're so inclined, teach him to fight back. Most bullies are also cowards and don't want to fight, so bloodying a nose or two keeps them away. I wasn't physically bullied much at all. I got into 2 fights my entire life and both ended with me in a half conscious rage chasing another kid around the playground. I don't know whether the perception that I'd go nuts on somebody kept me safe or whether it was the fact I didn't show any fear of them or what. Either way, I got the sense people knew I would hit back if they hit first. More often it was just trading insults and mutual nastiness, and eventually I got good enough at this that most people just avoided me. Of course, I was still an outcast and under a lot of stress from having the mentality of a cornered raccoon.
The schoolyard environment won't change much, as it hasn't changed much in decades, and public institutions lack the resources, the will, and the authority to exercise the sort of control that is required to prevent bullying. Your only choices are to get the kid out of the environment, or minimize the damage done as best you can.