Is it possibly a tactile issue? The feel of the shampoo, his hair, bubbles. Or not being able to see where he's putting the shampoo, I used a shaving mirror for my oldest. Mine would always forget to wash up front of his head and the mirror fixed that. For tactile stuff you could try gloves. If he's scared of soap in eyes, maybe some goggles.

My son will only wash his hair if he is wearing gloves (magic Minnies) so he doesn’t have to feel the shampoo on his hands. I also found a mirror helps so he can see what areas he has covered and which he still needs to do. I found getting him to smell shampoos at the store and pick one he likes also helps.

My 16 year old will usually shower fine on his own but we figured out he doesn't like multiple steps and doesn't like touching shampoo so he would skip washing his hair sometimes until we started buying the shampoo/body combos. He liked being able to wash his entire body with the same thing and will put it on his sponge and scrub his head with it as he is washing his body. Maybe something similar will help your son :)

Im still working on that one i have same problem so i fill up bathtub with lots of bubbles and soke clean i work on it every day but again i have a 18 year old son who doesn't have autism and had the same problem

We used the foam visor. I keep my sons hair short. We have a shower head that adjusts I use the setting where is comes out like rain my son also said the other felt like needles. Did you try putting shampoo on the sponge have him do it that way? I had my son try his own ways and see what made him comfortable. It took a while but he finally does it himself with out the visor. He still has some sensitivity but it's much better then it used to be.