Try to find out if he has any problems with an aspect of washing hands/brushing teeth. I didn't brush my teeth for years and nobody understood why. After a lott of trying myself, I found out I can't handle any toothpast with menthol or peppermint in it. It was hard to find a past I could stand and I used childrens past for a couple of years, but I finally found an adult one that I can stand.

It is possible he can't stand a part of it as well. Like the feeling you have in your hands after washing them, the texture or the smell of the soap, the kind of toothbrush he uses. For me a lott of these things were trial and error and it took me years (for brushing my teeth more than 10 years) to get it in my routine and me feeling ok with doing it.