I completely understand, Cancer is evil and upsetting and sometimes it doesn't turn out well :( Most cancer patients I have been in contact with need us to be strong for them, so maybe crying in front of them, especially if they aren't crying might not be the best option? Lol but on the other hand, maybe they NEED to cry with you. Be honest with him, tell him how upset you are, ask what he needs from you and tell him what you need from him. Maybe more understanding of his condition with help you :) Good luck!
Both my parents had leukemia and welcomed conversation. If you feel like crying, do so, and perhaps you will help the patient be able to talk about their feelings as well. Letting a person know you care about them, even through your tears, is always a good thing.
God bless you!