The thefts and all sounds like he might be using drugs. And drugs (both prescribed and illegal) can physiologically suppress the frontal lobe of the brain where certain immoral and illegal acts such as theft and even murder are controlled.
This boy sounds very dangerous to me. I would fear that he might somehow get the gun and use it on you or himself.
It might be time to get the legal system involved and make it a priority to get him out of your home and keep him out before he can cause more problems.
The Bible says we are to love others as we love Ourselves. There comes a time when we have to love ourselves enough to survive so we can love others.
Saying a prayer for your family.
smile