Hi M.
Being that it is summer, If your son is active I would say showering everyday, or at least washing up in the evening and showering every other day.
Usually around 4th or 5th grade most schools teach about puberty and good hygene and such, so now is absolutly appropriate for him to begin using deoderant. I know that they have some marketed for teenage/pre-teen girls, but I don't know about boys. He may need one not heavily scented though to aviod rashes and such.
Perhaps even a book on hygene from the library might be good if he does not feel comfortable talking about things like that with you. Some kids are very sensitive, but you don't want it to be another child at school sayign something about him need ing a shower. That would be awful, and the pre-teen years are hard enough! :)
Have a good summer!
Have a good summer!