If it were me, I'd keep him isolated and do daily water changes. Watch him closely and if he starts to show signs of distress, you might want to put him down to spare the fish unnecessary suffering.
It's really hard to know exactly what that is. It could be a gas bubble, bacterial or viral infection, tumor etc... As Byron stated, unless you see other fish showing symptoms, don't throw any medication in the main tank. Keep up with regular scheduled water changes on the main tank as well.
Good luck and I hope he recovers.