I would agree with the previous comments.
The oxygen that is available on the aircraft is for emergency situations only and not for general use.
The crew really need to know about your fathers condition prior to flight. This will enable them to keep a close eye on his medical status as the flight progresses and check in with how he is feeling regularly.
I understand that having additional oxygen onboard incurs cost and i know that this doesnt come cheap, but it really is in your fathers best interests to alert the airline prior to travel.