My mother died from cancer in December. I flew back to NJ to be with her a week before she passed. After 7.5 years in the infantry with one combat deployment, 7.5 years in law enforcement, it ws the most difficult mission I have ever had. I knew I was going back there to watch her die, and there was nothing I could do. I'm glad I got there when I did. She knew I was there with her. After hospice got involved, she was placed on morphine, ever increasing. The morphine eventually put her in a drug-induced coma. The only thing we could do were little things to make her comfortable or just hold her hand.
Every day is a gift. Just be there for him. If he is a person of faith or not pray for or with him. It doesn't make a difference if he wants to be a 4th Quarter believer, the Lord loves us all. Don't deny him that last chance to be saved. There are no atheists in foxholes or on death-beds.
Spend time with him. Just remember it is going to take a toll on you too. You should have someone to talk to, to keep your mental health in check.
God bless him and you!