oh brother it just gets better doesnt it, I just chickened out of the post below ...well I agree with Annie some hygiene has to be learnt I think you have to get a bit tough and explain he could get an infection.Does he do this where you can see it then ?
Wonderful. Aren't kids fun.
I knew a group of little boys who developed a backyard club. The club rules were posted on the treehouse door. One of the rules was "No picking butts."
I guess kids do it, but I wouldn't take a philosophical attitude about it if it were my son. I'd teach him about washing his hands and fecal contamination, and I would also get him on a vitamin and mineral supplement every day, in case he's got a craving for some trace mineral and finds it satisfying to do this for that reason.
Bleah, good luck.