Ok, so a miracle happened I guess? The swelling have reduced and the fish does not look like a balloon that is about to explode. No idea how to explain it, but yesterday the swelling was massive, the fish wasn't even trying to move, was just staying in one place, not moving. I was sure its doomed and that there is no way to help him. That was in the evening when I was going to sleep. In the morning I went to see if the fish is still breathing and what do I see? The swelling has reduced and the fish is actively swimming around the tank. Sure, its still a little swollen but not as much as it was for the last couple of days. I did not give him any meds, the only thing I did was a big water change (about 50%) exactly last week. Oh and I gave him hungry days. I did another small water change today.
Although the fish seems to feel better now, there is one more thing that concerns me. The fish's anus remains kinda swollen-like. I don't know how to explain it. I wanted to post a picture but was unable to catch it properly as the fish isn't in a mood to pose for a photo and the lights in the tank are broken. Maybe it will come back to normal in a few days.