You might also want to look into legal aid via your superior court and find out about emergency hearings. Depending on your state drug use in the child’s presents may be a cause to modify the visitation agreement.
If I was faced with this situation I would cut off visitation now and until it can be worked out in court. You have proof of documents that he forged in an attempt to cover up his drug use.
I have no doubt that his mother is in denial and enabling and I’m sure doesn’t see the cost of her behavior on your children. Like you are fighting to protect your children in a healthy way she is also fighting to protect her child but not in a healthy way. If she wants to visit her grand kids make sure she does it at your home with your rules that her son is not welcomed until things can be ironed out in court.