I've worked with children with autism and other similar disorders. I remember with one little boy, who was three, the parents cut his hair themselves (which is what you seem to be interested in, yes?), but they did it while he slept, because he was a good sleeper and was pretty much dead to the world when he was sleeping. Maybe that's a possibility for your son, until he gets more comfortable with it?
The biggest problem with haircuts is that the scissors get so close to the face and ears...you can let him hear scissors and see them, but it won't be the same as the closer view of those scary pointed cutting blades right by his face, or the sound of metal snipping against metal and cutting off part of him! I mean, that's scary for many children.
You might look around and see if there are any hairdressers in your area that specialize in children and adults with special needs. There are some out there...they are trained to make a haircut less of a problem for people with various disabilities, and since autism is one of the most common disabilities in children these days, that is one of the disabilities these specialized hairdressers know about. S/he might also be able to give you tips on how to make it easier for him in the future based on her education. Also, speak to your child's therapist and see if a plan can be started that will in time get him used to it.
It may take awhile, though, so haircuts while he's sleeping may be the best solution in the meantime, if possible.
Good luck!