Have you tried placing a weighted item in his lap while he sits in a chair (just to simulate getting a hair cut) while giving him something that he enjoys doing? May also add visual cues to show timeline "First hair cut, Then special treat." Explain
"why" it's important to get a hair cut (ie. hot weather/sweaty...)

Why does he HAVE to get a haircut? Does his school have certain requirements or something?
I would let the whole haircut issue go for now (if you can) and try to slowly desensitize him to someone touching his head. Once you get through that issue I'd try the haircut again.

I agree-the school cannot require a haircut (unless a private school). Restraint to perform the haircut will cause too much anxiety and is not a good idea. Perhaps he will tie it back. In the realm of ASD issues-hair is not a biggy.

Ben occasionally does this stunt. We keep him in a buzz cut because he wont care for his hair or allow anyone else to...super thick hair thats greasy & smelly?? No thanks! The noise of the clippers bother him...so we just wait till we cant take it no more...then we lie and tell him the whole family has lice...and tell him we got to shave it or do nit comb through it...works every time

That's a tough one. Don't have any advice. Just want to know how this works out for you.