I would suspect it has only really bothered him for the time you have seen the symptoms and prior to that it wasn't a harsh burden on his system.
It never seems fair when a pet goes so suddenly down hill but I suspect it is much better for them than a lingering incapacitating or painful illness, though its awfully hard on us.
I think feeling good and then a quick illness may really be easier on the pet.
This may have been coming on but not have been at a point where it really interfered with her life so he simply adjusted and carried on. Pets are much better at that than we humans may be.
Cancers can and do grow rapidly in some cases or very slowly in others.
I'd let him have and do those things he enjoys most that he can still enjoy.
I recommend the site http://www.petloss.com/ and the rainbow bridge story which may bring some comfort.
Hope this helps you!
Cat Health, Behavior, Care Expert
30+ years cat owner, rescue, breeding, study of behavior & health care