I'm thinking internal parasites. I once rescued an entire tank full of guppies that had been in deplorable conditions. After slowly getting everything cleaned up, I kept loosing fish. They would suddenly get extremely bloated- looked exactly like your guy- and die within a few days. After doing a lot of research I realized they had the white, stringy poops that are a hallmark of internal parasites. You say he is pooping... what do they look like? Like I said, white and stringy is a very strong indication of parasites.
That being said, I wouldn't bother with any external treatments. They really don't seem to be effective at all from my experience. The only thing that worked for me was medicated food (I believe mine was from Jungle Labs). The only problem is that it can be hard to get them to eat it, although soaking in garlic can help as it is an appetite stimulant. If it is parasites, that would also explain why the antibiotic treatment hasn't helped.