you absolutely have the right and duty to screen what filters thru to your minor child. TVs have parental controls. Movies have ratings on what is appropriate to what age group.
His father is not allowed to contact him, period, right? this g/f is just an extension of him, so NOTHING sent from him/her/that address, etc should be allowed thru to the child. seems pretty cut n dried to me......if you don't want to open and screen, then just throw it in the trash and leave your son out of it. this is ALL too much for him to deal with.