The way you deal with the bath issues could be easier to find after figuring out what it is about water that is so bad to him. I have a water aversion as well. Some of my earliest memories are of my "bath time problems". I'm incredibly sensitive to temperature, especially cold, and I'm in pain the entire time I'm getting dressed after a shower. To combat this, I have a heater in my bathroom and a huge fuzzy robe. The temperature of the water the first time I feel it matters a lot. If someone forgets to turn the shower head part off, the water blasts out of the shower head and tiny splashes of water hit me that feel like bullets. I cannot touch the water while its cold, nor can I touch the tile or porcelain of the bath tub itself. But, wouldn't cha know, I freakin LOVE jacuzzis!
So, my mother knowing nothing about SPD, did a few things because I was very vocal with her about what I didn't like. It took years to get it all right but this is what she did for me. She ran the bath long before I got in the bathroom so I didn't have to see or hear the water "being cold". Then she'd make a game: how fast can you get undressed and in the bath! I was very afraid of the drain (still dunno why) so instead of a real stopper she used this wash cloth thing to clog the drain. She also made sure to do the bathing as fast as possible cuz she knew if the temperature dropped more than a couple of degrees id feel it. I always had lots of bath toys, but they were dried and kept out of the bath to prevent me from touching them full of cold water. She used no tears shampoo and used a special watering can to rinse it out because I feared shampoo in my eyes. My hair was very long too and getting it brushed was a real task, so she had to use a de-tangler too. When bath time was over, she'd have the towel immediately ready and wrap it around me the same moment I stood up. A regular towel is not big enough, so she used a beach towel. Actually, it's the only towel I would use, thing must have been real beat up by age 9 lol. Our washer/dryer was in the bathroom for some reason, so she was able to put it in the dryer the whole time I was bathing and grab it nice and hot. Wow, my mother really was a remarkable woman (still is of course) for being so cool about all the extra work my brother and I made her do. (He was sick or in the hospital constantly until age 13-14, and I... Well, you know hehe.) She had me tested for autism twice as a child and she looked and tried very hard, but she never discovered SPD. She now says that she always knew that there was something very different about her shy, sensitive child and always went about my "quirks" like she did with the autistic children she worked with when she was a young adult.
I hope this helps to shed even the slightest bit of light on bath time issues, at least for temperature intolerant people. Oh and the clothes... My mom let me pick my own starting at age 3 I believe. She realized I'd only wear oversized t shirts and brightly colored leggings, so after a bit that's all she bought me. Also, not to be crass, but... In my adult life I've found it makes me just plain uncomfortable for others to "undress" me. It might not be a fabric, but the feeling of sliding fabric across the skin. Just a thought! Good luck!!!