What color is his waste? If it is white and stringy it is indicative of internal parasites. My guy looked similar to him without the white, but same swelling. You could try bumping his temps up to around 85* and giving a 2-3 tsp of epsom salt per gallon water. With my guy I just bought some Jungle Parasite Clear and put it in his water.
Before doing this I would really recommend you find some of his droppings. If you can't find any he may be constipated and you could still do the epsom salt to try to get him to go. Bloodworms that are freeze dried can cause constipation and that could cause the swelling. Either way epsom salt baths are effective against both parasites and constipation. To be effective agianst parasites it needs to be combined with the anti-parasite meds though.