JUst from one chicken experience.. so others may know better... keep him in sight of the others if possible and when you know he's better, pop him back just as they're settling down for the night. Keep an ear on them in case of fighting and be sure to let them out early the next day to go about their business to give them all space. As they all know each other you should have less probs than introducing a totally new one
Glad he's better, its always a worry when they're 'off it!'