My asd DD could not stand bubbles. Hated hated hated soap, shampoo etc. Her hair has always been long, waist length long. She struggled with it so much, either she would come out of the shower telling you she'd washed her hair when it was obviously still dry in places, or she'd do it but only patchy clean, or not rinsed properly.
Even at 16/17/18 she preferred someone else washing her hair for her so she didn't have to do it. Leaning forward over the bath using the shower attachment fully clothed, not a problem. If it made her week a little bit easier, it's 10 minutes of my time and lets her get on with her week without being stressed out about her hair or the thought of having to wash it. Short hair I'd imagine is a 2 minute job. She knew how to do it, just hated it. I hate spiders, I know how to get rid of them if I have to but if there is someone else I can ask to help me, I'm gonna take that option.
As long as he knows how to do it himself, I don't see the harm in you helping to make sure it's done properly, if he's ok with that. Do it while he's still clothed so he has his privacy, treat it as a separate thing to showering/bathing.
DD is now at uni, with hair that is now bum length, and I'm really proud of the things she is now doing for herself. Because she's had to. She still hates hair washing but is doing it once a week, on a set day, and doing it properly, because she wants to make a good impression to her new friends. I wouldn't worry about helping an 11 yr old with his hair.