He apparently called their schools and the police, so he did everything possible, he just asked us if we had any information on the whereabouts of the former tenants (as the letters were for them) so we could notify them. We've contacted the real estate and left the matter with them.
Luckily, we don't have any mail coming to the house address, we've had a post office box for some years now, but, it's sad that kids are doing this.