Deb,
His mother may very well be everything that was behind his drinking from day one. Dysfunction breeds dysfunction and perhaps the apple did not fall far from the tree. They usually don't.
Don't accept the blame they put on you. You cannot control another person's behavior, and you did everything right.
I like what was said above pointing out that his mother did not fix it for him, either. And the co-worker, what did he do? Why was it YOUR problem to fix, and yours alone? Answer: it wasn't. It was only his to fix, and he did not rise to that. You are not responsible for any of this.
(((((Deb)))))