That reminds me VERY much of what happened to my guppies. I can even give you pics if you want although they are a little blurry. The same thing happened...they would bloat up and right before they died they would develop open sores on the stomach area. Turned out that they had internal parasites. I'm going to say the same thing that I just wrote to another person. Look at the feces to see if they are normal colored or white and stringy.
I don't think that's constipation because I'd think that by that point the severe amount of waste building up inside him would have killed him. Tumor could be a possibility but this looks too much like what happened to my guppies for me to think so.
I used Jungle Labs Anti-Parasite medicated food to cure my fish. I tried many meds before that and none worked so I was amazed when this one did because I had pretty much given up hope by then.
Here's the pic of one guppy: You can see how bloated he is and I circled the sore which is hard to see but you can see the red spot.