Hi,
It is difficult to predict the course of cancer based on what information you have been given. If he seems to show no signs of the disease (not even any weight loss) - perhaps the best course of action is to encourage him to seek a second opinion if he doesn't want to see his first doctor. He may not tolerate one kind of chemotherapy - but it is possible that a different kind would be acceptable to him.
If you cannot get him to do this, then it is best you discuss how evading this subject is making you doubt his sincerity.
Stay positive. If he really is ill, bear in mind that he may be in a state of denial even if he actually agreed to some treatment in the past. The most you can do is encourage him to open up - but unfortunately, some people never go beyond the state of denial.
if he realy cares about you he would listen to you and do something about it. if he's letting nature take its course that's not good. you have to wonder if he's telling the truth.