I am a cancer patient with a pretty grim prognosis from Stage 4 colorectal cancer. I am 50. I have a wife and a 12 year old son and 15 yr old daughter.
I believe in God as I am a Deist. However I don't find strength in prayer. I find strength in people. My friends and family have helped me more than any prayer. I have found peace of mind by talking with other cancer patients, doctors and nurses.
How incredibly cruel that a 21 year old young man with his whole life ahead of him and a loving person like you in his life has to go through this s**t. The best you can do is stay positive, don't give up hope and support him in every way. For yourself, find others in similar situations and communicate. Talking about it somehow is a pressure valve that makes it more tolerable.
All of us are affected by this and we will be forever changed from the experience. There are few positives but there are some. We will be more empathetic. We'll learn a lot about ourselves. We will have completely different perspectives on so much of life like illness, healthcare, pain, God, hope, death etc. etc etc.. Best of luck and hang in there.