You can't play the shoulda, coulda, woulda game with yourself. You brought him in to the vet as soon as you realized what was wrong. Animals are masters at hiding when they are not well. It is a survival of the fittest thing. In the wild the sick would be left to die. It is just their natural instinct to try and hide when they don't feel well.
I will be sending many positive thoughts for your Jake. Just keep strong and give him lots of hugs of course.
Denyce