Thanks for your advice. I have to resubmit form.5669 and I will mention about his illness there when I submit.Has your father had his medical yet? If not, your father should mention at that time and include what medications he is taking. If he has his official diagnosis on an official document from the hospital he could submit that to the panel physician. He could also submit his official diagnosis, treatment and prognosis from his oncologist when he does the medical to try and speed up the process since extra information is often requested.
I can’t remember if you had to include whether you have a serious illness in the application. If you do and he said no I would also update his application via webform.