I am another "no" vote.
I do not think it will have the desired outcome and could easily have the opposite of the desired effect. The BF and your grandson are probably operating on the same cognitive level, that of a young teen, despite the differences in their age.
Not getting what you want, when you want it, regardless of the implications, is common at that age and regardless of age, when addiction is involved.
For the time being, it's probably easier to make you the scapegoat than face what's really going on.