Did he have an itchy bum?
Simply ask him why he plays with turds
tell him secretly.. then speaks to him...
How can you say it is only from his fingers? Why can't it be from his entire palm or hand or body? If it is from his fingers from which finger particularly? If it is really and only from his fingers ask him not to use his fingers for any work directly. ask him to use gloves.
Oh..I feel sorry for the situation.
Nail or skin infection.