I would be freaked out, i don't like nobody in my space that i did not invite and i got a very, very, big space.
He mentally challenged and by law should be monitored for his own safety and well being as well as those that are around him, i can't believe they just turn him loose on his own, who knows whats gonna happen.
I would call social services and make a report, The bigger he gets the bigger the problem is gonna become.
It's sad that none has taken the time to teach him right from wrong but at this age a normal teen is just starting to deal with the facts of life and some of get pretty wild.
Hope you get this taken care of it is a very scary thing, he may get shot one day walking into the wrong house at the wrong time, if someone walked into my house and i did not know who they were or what they wanted and felt fear, well it won;t have a happy ending if i have no choice.