Talking with a person with cancer, first of all, on abstract topics, and if you calm him down, it will annoy him even more, he is very scared by the news that someone is already sitting in him!
In this situation, IMHO you should keep your thoughts to yourself a little. It is especially important to give a person pleasant moments of life and make his life easier as much as possible, but at the same time not so that the person would somehow think that this is all because he is sick, or because then he will not receive anything. This is all purely my personal opinion, but if someone supports him, that is, you have new food: think about the fact that you may not know that a person has cancer or that he will get sick, but at the same time your behavior is him such to himself. Dinner is served.
Thanks for the question, good luck.
As with a patient with any serious illness. Your question is not about communicating with a dying person. Cancer is a disease that is often cured, although it is difficult to treat, usually expensive, and the risks are high.
A person is already supported by the fact that he is not alone, that he can, IF he WANTS, talk with someone about his thoughts and experiences. And this someone can tolerate it, does not run away and does not shut up his mouth with cheerful exclamations.
Talk - on topics that are interesting to both of you. Or they are interested in him - if you just want to take care and support.
This is likely to be soothing.
And "nothing, everything will pass!", If you are not his doctor, in In a large percentage of cases, a person can perceive it as a devaluation - not of a disease, but of himself and his experiences. - Although it is important for someone to hear this. - All people are different, both healthy and sick.
In a word, the main advice is not so much to "calm down" as to be with him and respond to topics that are interesting to a person, excite him.