Use medicine (I recommend trying Methylene Blue). Isolate him in a separate tank and then use Methylene Blue. Use aerator to supply oxygen to him. Give him medicine pellets if he can eat. If he doesn't, don't give him any food. Change the water 25%-50% (if the tank is very very small like the tank used for solo bettas, do 100% water change) every 2-3 days (if you don't use any filter, do it every 1-2 days). Put half dose of the recommended medicine dosage every water change (or the right dosage, your choice).