My mother was in a nursing home for years, and there are different cleaners that they use in that situation besides soap and water. You'd use the cleansers first, get the bulk of the stool off, and soap with water as a final step.
I worked in a hospital as a tech. In your situation, we used hand lotion to get the mess in the hair out. You can even warm up the bottle a little, so it isn't so cold. Then a little soap and water.
We also used the PLAIN Barbasol shaving cream in the can. That worked well too. You don't need much of the cream to get the job done. (Google to see if they have it in your country, or what brand would be similar)
If your son goes to physical therapy, ask them what they would recommend. Your problem isn't that unusual.
Hope this helps.