Hello and welcome,
I apologize that no one has responded to your question sooner. Different experts come online at various times. I just logged on and saw your question. My name is ***** ***** I’m a biologist with a special interest in fish. I'm sorry to hear of this incident.
You're right that this does not sound like a swim bladder disorder. As for feeding twice a day, I do that, too, and have for years.
If the original cause of this lump was an injury, there are two possibilities for what it might be. One is an abscess. However, if an abscess was present for months, chances are good that the infection would have spread, leading to additional symptoms. It could also be internal scar tissue, which would not cause symptoms.
There are two other concerns with swelling like this; neither related to an injury. Kidney disease can cause swelling on one side. A fish can go for some time with that, but eventually it would cause illness, usually dropsy symptoms. Another possibility is a tumor. Many tumors in fish are benign. Unless they interfere with eating or swimming, a fish can live out its lifespan with such a tumor.
The only way to get a certain diagnosis is to take him to an aquatic vet, who can palpate the swelling and determine what it is. Unless it is an abscess, there won't be a reliable treatment. If you want to do that, this link will take you to a directory of fish vets. You'll want to enter your state, check the private practice box, and the ornamental aquatic animals box.
If you don't want to see a vet, you can try a medicated antibiotic food. I would also add aquarium salt at the rate of one teaspoon per gallon of water, and StressCoat according to directions on the bottle. If the swelling is related to an injury, these steps may help.
I'll be glad to discuss water quality with you. Just give me the details - test results, etc. Thank you.