It is unclear from your question how old your son is, age is a factor on how you would talk to them. I am also assuming it is your child who is ill, and not you. This is a very very hard time for all of you. Tell your child that cancer is not a death sentence, and there are lots of treatments available and in the pipeline. Let him know you are there for him when he is ready to talk, and that anger is part of journey.
I would ask the oncologist for a referral to a therapist who specializes in treating individuals of your son's age with cancer.