Yup, I'd just tell him you're done. If he wants a babysitter, he can go to detox where medical professionals will be happy to bring him safely down and refer him to resources if he's ready to be done with drinking. But it could be years/decades till he's at the point where he would actually follow through on that.
Be prepared for him to try to manipulate and guilt-trip you to get you to come back. Have a plan for yourself to resist any such efforts.