Am I right in thinking the entire group is new and were all bought at the same time.
If so you may as well put him back in the tank. Being moved from the store to your tank is incredibly stressful for fish. Before that he was moved to the store, either from the peaceful river where he was minding his own business a few months ago or from a fish farm. Another very stressful experience. Then he went through an uncycled tank. So if he is weak he has everything going against him. But if you did buy them all together any disease he has the other fish will have too. And since they seem unaffected it is most likely just the accumulation of stress he has been through.
I suspect at this stage he will either live or die, and nothing you can do will change that outcome. Being in a tank with more of his own species is likely to be less stressful than being kept in isolation.