Prostate cancer at 76 isn't that unusual. Sleeping with a mask probably means he has to sleep with oxygen mask due to something like sleep apnea. It's nothing really. Not a big deal. Not like he is 20. It's wrong of his mother to inform you but at 90, I'd say as smart as she sounds her sense of boundaries might be off. As people age they often put foot in their mouth.

Overall what does he tell you about all that? What does he suggest? How often does he call? When are you two meeting again? I would just worry about how he is pursuing communication with you. Don't worry about the mother