Good morning dear Gerda. Finding out your mom has cancer is a lot to take in at 48 (I speak from experience), let alone 15. Crazy hormones racing around, not sure about life yet, one foot in childhood, the other creeping toward adulthood. He needed to know. He now knows. Likely he will process it in incremental stages. A gentle reminder that our interpretation of the way someone looks at us is not necessarily what's actually going on in that person's mind at the time (we are all guilty of this).
Remember something: YOU are all he has. Please make sure he knows you're not going anywhere. Not sure how you do that, but I trust you will know. He might have a lot of anxiety that something will happen to you before he turns 18 and he will have to live with his dad. He might be upset to learn this after the fact rather than in real time. We just don't know. Keep as much communication open as possible and remember that he's still so young and going through his own changes that he is likely not completely aware of or able to articulate.
Regarding your H putting your daughter in the middle and triangulating through her - no words. We have no control over anything but our attitudes and what we bring to the table, as you are well aware. You do a beautiful job of keeping your side of the street clean and protecting your children as much as you can. The rest is up to God. xoxoxo
M 20+ T25+ S ~15.5 (BD) BD 4/6/15 D 12/23/16
"Someone I loved once gave me A box full of darkness. It took me years to understand, That this too, was a gift." ~ Mary Oliver