Do you think, as a Father, that your son might be in any sort of danger from this man, nailit?
If dangerous when on drugs, who knows what he is capable of, when off his head.....you said they fell out....how were things left?...
I would keep trying to contact him via FB....to have a conversation, as you say....find out why he is sending gun pictures to your son.
You dont want to involve the police, you mention, for various reasons.
Not a good start to 2018 is it, nailit?......but unfortunately because of the company your son has associated with, over the years, you are never, in the foreseeable future anyway, going to be a worry free Dad.....and your son is never going to be out of some sort of danger. I, as usual, wish you all the best....nothing more to add.