Our youngest son hates being bathed, let alone having his hair washed. Now at the age of 5 he will sit in a shallow bath of lukewarm water, as he doesn't like the water temperature to be warm and has recently accepted us using 'Incredible Hulk' body wash on him. I am forever looking for Superhero branded products to encourage him that he needs to be clean like Iron Man or Batman etc.
Washing his hair is a totally different ball game as he will not let you get his hair wet. The other day I suggested we use 'Incredible Hulk' body wash on his hair to clean it and before I knew it, he had squeezed a large amount of green body wash onto his hands and put it straight onto his head. I already knew what was going to happen - series of screaming and tantrums when I had to wash it out. He protested that I couldn't wash it off and instead wanted me to hold him whilst he laid back in the bath and got his hair wet. I gave it a go, but once a millimetre of his hair touched the water, he sat bolt upright and was screaming he had to get out the bath as his hair was wet! It ended up with me getting soaking wet whilst I held onto him and gently poured lukewarm water over the back of head to get as much of the body wash out of his hair as possible. I've tried using a flannel or a towel against his face so that he can cover it and ensure nothing is splashed on him, but he doesn't like not being able to see and so not knowing when the water is going to touch his head. He will not tolerate going under the shower and will not lean his head against a basin so that his hair can be washed that way.
Today I went to see our hairdresser who has built up a lovely rapport with our son. She often cuts hair for ASD children and has a family member herself with ASD, so has a good understanding of autism. She has suggested to me that on the evening that she works late each week I bring our youngest son in and she will have a go at washing it there. It may not work, but it's worth a try.