Boys at any age play with themselves.

I would ignore it.

If its really socially inappropriate maybe you just need to comment "better save that for when you are alone little buddy... its not really something to do in public".

I have heard that in Japan, it is accepted that boys play with themselves and no one discourages them. So 10 yos are always walking about with their hands in thier pants. As they grow older... they learn to be more discrete... its just natural.

Cat.