Thats not the stomach, its the gallbladder
The stomach is attached to the loop of intestine that is directly below it. Not really knowing about the internal workings of this fish its possible that it has something in its stomach that was backing things up. The gallbladder does look slightly larger than it should be. The first photo of your fish with the vent (anus)protruding is a good indicator that something is up, was it like that in life as well as death? The liver also looks pretty pale. Sorry I wasn't closer and I'd take some samples and have a look see whats up.
So sorry to hear about your fish, he was truly magnificent
