I get the feeling there isn't another male around the house?
My brother who has special needs doesn't really understand the concept of being dirty or clean unless he's like, covered in mud.
He may have a problem with touching and cleaning his own body due to the fact he has hit puberty? My brother had to be shown how to wash himself because if he hadn't of been shown, he wouldn't of done it properly. He thinks getting wet means he is clean. And that was when he it puberty too because before that he never bothered closing the door so my parents would walk past and be like "Make sure you wash!" and he's just put shower gel everywhere which works haha
Could you try running him a bubble bath?