You cannot save him. I don't mean to sound harsh, I just want to say that in hopes of you accepting that and hopefully getting some relief from that.

You also can't make him realize he is terminally ill. He may know it but refuse to outwardly acknowledge it. His medical treatment doctors I'm sure have told him.

Could you go another direction, and suggest to him that, given the seriousness of his illness, it would be a good idea to give you guardianship or custody if his son, who needs a caring and permanent person in his life. That conversation should not be about his drinking at all, just about his son's needs.

Another separate conversation could be about his future plans. With this serious illness, I would ask him if he should become incapacitated, does he have a preferred facility where he would like to go to be taken care of?

These kinds of calm and matter-of-fact conversations can be relieving to him and to you, and might help him see beyond the drinking issue, which is understandably very frustrating and heartbreaking.