Dealing with cancer is never easy under any circumstances. I'm 22 years old, but have already lost my father and a sister to cancer. Living through that had to be one of the hardest things I've ever encountered and dealing with the caretaking is no simple task, but I believe that leaving the person you care/have cared about during this time is something you'd likely hold against yourself forever, especially if the person passes this world. They are not themselves while under the treatments and pain of dealing with the disease, I know my Dad for example tried to hide things so I would go away because he did not want us to have to take care of him and felt ashamed at times that we did.
My grandmother in her illness would go into stages where she would suddenly snap at any one of us and scream that we weren't supposed to be anywhere near her, not even knowing who we were.
Though it was extremely difficult to handle situations like this, and may be considered selfish on my part, I would not trade one single day I spent with them, not even the worst.
I have life to go on living now that they're gone and can gain back any time lost, but I'd never be able to gain back one minute spent with them again, and I finally did break down with my grandmother in the end, not seeing her for a while... That is the one mistake I know I'll live with and regret for the rest of my life.
I'm very one sided in this situation, sorry, but I would make sure you could handle it if he were gone tomorrow.
The only other thing I can suggest is contacting hospice for help because you do need a great deal of help through this. Also, start going to church regularly, it may not sound like something, but I found that to be the best support group I could have ever asked for. Shoulders that didn't mind you crying on them, didn't judge your thoughts, just listened and tried to give any guidance they could as well as were all the first ones to jump in and help.
Don't be afraid to accept help from anyone willing to give it.
Small outings to something like a garden or butterfly exhibit, anything that might reflect life and hopefully help remind him that he is still alive so that he has a desire to live his life to the fullest. May also involve getting him to meet someone young who is fighting the battle, I find that they are often a great inspiration to the grown ups fighting the battle because of their extreme courage they tend to show.
That's just my opinion however, I am only 22 and in no way an expert.