Have you asked him why he doesn't want to?
The suggestions I have are a social story, about hygiene, or him using a disposable glove, if he's worried about getting dirty.

Hi my son hasn't yet had a diagnosis it took till he was three just to attempt the to get his hair cut, and his hair was so long and curly was in a pony tail it took alot of attempts finally when he was four we had success but he has to go the same salon at the same time with the same lady to cut his hair. Harry is six now but won't wipe his own butt. It's the first time I've read someone else mention the hair cut problem.x

Definitely a difficulty here. My B is too busy to wipe. I do it of his dad. His BM’s are on schedule so we usually catch him in time. Sometimes he doesnt want any part of it but we make it a priority. Sometime a reward system works. I recently started usuing social stories for hair cut and it worked wonderfully. We are in the process of implememting them for everything. It really helps with these new aggression behaviors as transitions seem to be a huge trigger.

I am going through the same issue. He is 9 as of Monday and he refuses to wipe. Just got to keep a lot of bleach on hand to keep the undies clean. He will let me wipe him but no one else. And of course I'm not always there to wipe him😕