I don’t think there’s much you could have done to prevent the cancer, sadly, it’s still horrible but you shouldn’t feel guilty about not having the blood work done. The decision to put a pet down is never an easy one, either, you feel like you’re damned if you do and damned if you don’t.
You sound like a really caring person, the intent to do everything you could for your boy was there and even though you couldn’t do as much as you wanted, it still counts for a lot.