Yeah, unfortunately there's not a lot you can do until he's ready to stop, though you can certainly express your concern and your hope he will decide to go back to AA. If your mom knows any of his old AA friends, it might be worth giving one of them a call to talk to him. It's very hard for someone to come back into AA after "going out"--there's usually a lot of shame and embarrassment, though everyone will be understanding and welcome him back. It happens to lots of people--sadly, even those with a lot of time sober. I've been sober for 8 years, myself, and it's always important to remember that we are all just one drink away from a relapse.
You, your siblings, and your mom would all probably benefit a whole lot from Al-Anon. It was a lifeline for me when my second husband went back to drinking after almost dying from it. I think you're smart to recognize the need for support for yourself.