At this point, anyway, it appears you cannot "be" anything in terms of a relationship with this man, except as it concerns your parenting obligations. He needs his own support system, without you in it.
I am exceptionally fortunate that my kids' dad was sober for fifteen years when we divorced. Though we went through some rocky times during that process, we came out of it good friends and we are able to be emotional support for each other in ways that have nothing to do with the kids. That is the EXCEPTION. I don't think it is possible or realistic for most people to maintain anything more than a civil relationship where you cooperate regarding the children.