Considering that chickens are social beasts that like to be with other chickens I would leave him with the others.
The odds are that your bird will not have any ill effects eating the same diet as your other birds. I also don't think that the costs and hassles associated with setting up housing for one bird are worth the potential losses even if the worst were to come to pass and the bird were to die.
Also, the bird MIGHT be a red sex link packing peanut roo. The ones I have had grew super fast the first few weeks, but then slowed down dramatically...