You're doing what's right for your son and your for your own mental health. You haven't taken away anything from her - remember that- she has given it all away to booze.
If she chose sobriety and real recovery she could build a real relationship with her grandson. That is her choice to wake up to. That is the same choice we are all given!
Doesn't make it easy, or less painful I know. A therapist told me once "Guilt seeks punishment." So finding ways to let go of the guilt will keep you from making rash or unhealthy decisions that will only end up punishing you...AlAnon helped me with that.
(((((hugs)))))
b