I was bullyed as a child, so I have some idea what he's going through.
I was always told to just ignore what they said, but that didn't work. It still rattled around in my head, the things they'd say, and they lasted until I healed. I was a sensative, emotional, caring and compasionate kid. And they walked all over me, and it hurt badly.
I'd say this- teach him that you and the adults around him DO have the power to change his world. I didn't think my parents could do anything- I didn't think they'd be beleived, because I didn't thiink they'd beleive me and my parent's info was strictly based on what I said, so why would they be beleived? (I hope that makes sense...) I didn't want them to call the other kids' parents because the bully would retaliate on me at school as soon as they got the chance. But I was wrong.
Telling my parents, right away and in full detail would have been one of the best things I could have done. And I do beleive that, working together, we could have changed my world and maybe saved me some pain. So teach your boy that the adults around him have the power to change his world. Then he'll tell and keep on telling until he's beleived. And he'll tell you too.
in this case, I think you need to contact the parents. And if they don't do anything, there are other options. If he's being hit, and the bully is over 12 (here in Canada, ages vary country to country), the bully can be charged with assault. If he's under 12, and the parents are doing nothing to address the problem or refusing to take it seriously, you could consider contacting your local Child Protection service. They would reinforce how inappropriate the behaviour is.
And considering your son has a disability, raise all the fuss you can on his behalf. It is WRONG to discriminate based on disability, and that's what this bully is doing. Talk to the principal, the teacher, the school counsellor- everyone who'll listen. Be his advocate, your good at that!