It looks like bacterial infection to me. Is he in the quarantine tank now? Is your tank heated and filtered? remove the carbon from filter if you have one (it will remove all medication if you keep it in and your meds won't work). I would suggest to try antibacterial food and/or another cycle of antibiotics and keep him comfortable. Keep water stable and clean + give him some place to hide (something he was familiar with if you moved him to quarantine tank). You may also try to cover the tank with towel to help him rest. Depending on the size of your tank (it looks like he is in a glass bowl right now), you may also want to lower water level so he won't have to swim to hard for air. Keep feeding him and vacuum uneaten food asap, to keep water clean.