Molly,
I will never understand why women stop grandparents seeing their grandchildren because of a grown childs behaviour. You are not him, its not your fault, and its not fair to lump you in the same group.
Maybe you could talk to the mothers to explain how you feel and what you are going through, I can understand why they dont want the children around your son, but you are not a drug addict. They may feel as though you are taking his side and accepting his problem, make sure they know that you are not accepting of it.
Partners of drug addicts are damaged, they’ve probably been through more than you realise, and they may be scared that you are trying to allow your son to get to them through you. Just make sure they understand that you are at the end of your tether and that you want to have a relationship with your grandchildren, with a sober son, or without him if he cannot be that.
Sending love x