Well, I can just say that sucks. As an employer, I couldn't imagine not being supportive, if anyway possible. From an emp loyee's point of view, I had one that was good to me when I was first diagnosed the first time several years ago. I went to work for them and 3 weeks later was in the hospital getting treated. I was being paid salary, and I was a key employee. They still paid me the same each week no matter if I was there all week or out for two days. One the other hand, I was there everyday unless I was attached to the toilet getting sick, or in the hospital. I was the company poster child for a positive mental attitude, and you can work if you don't feel good.
You need to find a way to make him realize that yes he was good to you, and you wouldn't expect anything else, nor would anyone else. Firing someone because they are sick is a touchy, mostly illegal thing, especially someone with cancer. Most companies wouldn't want anyone to know they canned someone with cancer, for no other reason. You just have to be sure, there is "no other reason".
Best of luck to you, and I'm glad you doing well.