Oh I am just sick reading this. I remember when your son went out there and your father was trying so hard to help him. How on earth can a grandson do that to his grandfather? I know, no answer. Or no good answer.
Like someone else mentioned, I would urge your father to ask a lawyer if there is any loophole to get difficult child out sooner than later. There may be something in there about the fact that he was gone to jail and hence, out of the home, so he may have been able to be legally evicted. Police may not know all the in's and out's. I would think that if your son is doing anything illegal that your father could charge him and that would be grounds to have him gone asap. Of course, that is just something that SHOULD make sense. The law doesnt always make sense.
If you are like me, you probably feel really guilty about what your difficult child has done to your father. I always have. Please know that you cant be blamed for this.