I am so very sorry. I know how very difficult all this has to be for you. You have done the right thing in leaving.
To address his mom, quickly... people need scapegoats. Someone they can blame for all that's happened. And instead of accepting that her son is an addict and confronting the actual problem she blames you because its easier. Hopefully for the sake of her son, she'll get over it and see what's going on and that you had NO control over the situation. Let alone any fault.
A lot of us go through a period of, why didnt he fight for me. But if you're going to be the main road block to his addiction - there's your answer. Addiction is all consuming and for some it's easier just to walk away so the fix will be easy...But then, who knows what's in his head.
You tell your son in the simplist of ways. Mommy and daddy aren't happy together so daddy went to stay with gma. Then continually remind him that you both love him, that he'll still get to see his dad, etc. Simple is best at that age.