1 c. borax
1-2 t. pure turpentine
1 t. sweet orange essential oil
1 c. ground soap
With very clean hands, work the turpentine and essential oil into the borax until there are no lumps left, then work into the soap. Keep it in a wide-mouthed jar or tin that's easy for him to open when his hands are greasy, and which you won't mind getting black on the outside. Don't forget to put a nail brush and pumice stone out with the hand cleanser.