I have just been dealing with a similar situation with my AD and my parents who live in another state--no dealing & crime, but addiction & its consequences for sure.
It is enough that you are in contact with your parents and on the same page. Keep it that way. As for what you tell your son, it probably doesn't matter. If his grandparents keep their boundaries and you keep yours, he will figure it out pretty quickly. No need to tell him you are on to his schemes. If you want to be in touch with him, maybe just text him and tell him you love him and pray for his safety and health every day. That is what I do. Where ever she is, whatever she's doing, she knows I love her. I can't control what she does or if she chooses recovery, but I do know I will always love her. That is really all I have left.
I completely understand the urge to contact him about it, though. Just let that pass through you. That is part of your journey. Loving him is all you can do now.