I had a tiny Tiger Barb look that way. He was always much smaller than the others. He developed a sunken belly like that and looked weak with poor appetite. Sunken belly is known to be extremely contagious. I removed him and euthanized him immediately. None of the others were ever affected. Usually it just happens to a weak fish. If removed quickly enough, it usually won't spread. It's usually caused by a parasite.
Once the fish is in this condition, it often does not recover. If you cannot force feed the treatment INTO the fish (these fish are too small to do so), then the best option is usually to euthanize the fish. There isn't any point in allowing it to suffer like this. He won't recover at this point.
It isn't likely fish TB. More likely an internal parasite that the fish cannot fight on his own that has taken him down. Please euthanize him and keep an eye on the others.