I saw your update and my heart aches for you. I agree with another poster who said not to mourn him while he is here. Enjoy the last precious days. Hug him, hold him. It sounds odd but you will comfort each other during this time. And even though your son is a young adult, please make sure he understands it is OK to mourn. He is at a difficult age. Grief is grief regardless of age and it hurts.
We struggled a year with health issues with one of our dogs. We put our beautiful dog through hell to keep her breathing. It was wrong. When we got the liver cancer diagnosis for our oldest dog last fall, we brought him home for a final night, loved him, said goodbye. With a sad, but peaceful heart helped him pass. Know that what you will grant Hunter is a peaceful and dignified crossing. You end his pain, but bring that onto yourself. Stay strong, but have a peaceful heart. You, your son and Hunter will be in my thoughts.