This is a tough situation. My wife had some issues when we met and she passed away as a result. I went into it knowing she had a past history of cancer. I do not regret it, but it took a lot out of me. In a way, I believe God brought her into my life so she would not die alone, that she would have support.
He is facing serious issues. You cannot and will never be the center of his thoughts. He is hot and cold as he moves through the stages of disease and mortality. You just left a relationship and you deserve to be cherisedand loved.
He cannot really give now, he can in spurts based on what you are saying, but I get the feeling you want to be the center of his life which is normal in a normal relationship. See this as what it is, a nice time together, support and some fun and hard times, but based on your comments this is not a good basis for a long term relationship, you are not equal coming together as one.