Ds1 is 13 and wouldn't set foot in a hairdresser's/barber's if he so much as heard the clippers. To him the noise is painful and I would imagine having it actually touching his head would be agony. The other sensory issues mentioned by NoReallyYouGoFirst were also a problem, as was his fear of unknown tv programmes - which isn't great as most barber's locally have the TV on, even if it's only sport.
He was extremely head sensitive when he was younger, we couldn't wash or comb/brush his hair without histryonics (sp?) let alone cut it.
When he was little I trimmed his hair myself. Started off just doing his fringe while he was asleep and gradually worked up to him allowing me to do a bit at a time. We were then lucky enough to have a local hairdresser's open in the village and they were so lovely with him. The first couple of visits he just watched his brother have his hair cut - the lack of clippers buzzing away definitely helped him cope with the environment, then eventually he let them first comb his hair, then trim the ends.
He has always had long-ish hair (think surfy style) though, as he still couldn't handle a full cut - until last school year, when sadly our lovely hairdresser decided not to come back from maternity leave and being in secondary, we need to start keeping his hair collar length. We had a disastrous trip to a barber's in the next village - dh had to almost grab the guy to stop him using the clippers, even though we'd arranged in advance that he wouldn't and poor old ds came out looking distinctly scalped - as did ds2. 
This set him right back and he flatly to have his hair cut anywhere for months on end.
Fortunately, last month we tried one on a recommendation from someone ds respects and they were great. They booked us in at a quiet time, turned the tv off and didn't use clippers on anyone for the whole time he was in the shop. He's still not keen on going back, but says he would rather go there than anywhere else.
If you can't locate a SN hairdresser, try having another look at where you take him and see if you think the environment itself is having an impact, then see if you can go back a few steps, take him before he needs to go and just get her to snip off one small bit of hair, then leave, next time a little more and so on until he can at least have a reasonable trim. I would think the clippers are probably the largest part of the problem, so eliminating them and showing your ds that haircuts are no longer scary/noisy might help.