I agree, your son is probably afraid of losing you. Reassuring him that you are doing well and cancer-free now, and explaining that you didn't want to worry him when everything was going on with his dad, should help.
But also - now that you've let that genie out of the bottle, I believe you should tell your daughter too. I don't think a teenage be can be trusted not to blurt something out in the middle of an argument with D11. Take the same approach with her. Reassurance.
