Due to the swelling being uneven on either side I would say that is it an obstruction of some kind, It could be a tumour that is or not organ related or it could be
severe constipation. Constipation can be cleared with the lack of food and pea feeding as already said but if the blockage is so set it place it will not budge regardless of the peas but what will happen is what is called "Overflow". Overflow is when the passing of the waste has no choice but to go pass the blockage so will push the lining of the bowel to its full extend in order to pass by the blockage.
1) Is he passing any waste?
2) What colour is it if he is?
3) Can we have a birds eye view of him please?
What ever the problem, it seems to be pushing the organs to the back end of him, giving the purple patches as you can see. This will in time stop any waste passing through. The outcome of this could result in major organ failure and severe dropsy, or a very bad case of swim bladder.
If i was in this position I would keep his water extra clean, I would tub him up and do water changes at 100% daily, 100% will not harm him if he has no filter in the tub and keep the water level to about 3 inches. I would also feed brineshrimp that contains garlic, this comes in frozen form and a good LFS will stock it or can get it easily. Do not use fresh garlic as the measure of the garlic in the frozen food has been measured to the perfect requirements. The garlic will boost his immune system very fast and will help a lot, the brine shrimp tends to be able to help any blockages soften and in some cases works better than peas. I condition females with brineshrimp and garlic only. Due to the over feeding needed this food is less likely to cause constipation during this period and keeps the female regular so to speak and the immune system boost helps her to recover from a bad spawning much faster.
If this does not help immediately (2 days or so) then i would persist with this food for atleast 2 weeks as if it is acute constipation it will need time to soften and pass. If this doesnt help then the problem will be a cyst or so far advanced constipation that its beyond help and then you can only make him comfortable until the end nears. If you choose to cull then thats your decision on the matter.
Good luck!!! and try to get the birds eye view please
