The problem is you're taking him away from his parents who need to feed him and teach him things, the parents stay with their babies for a little while after they fly. Even after they are flying the parents are still feeding the baby. The best thing you could do for him is not interfere and let nature take its course. If he's not hurt then he needs to back where he was, its his best chance. If he is hurt then he needs to go to a vet or rescue center where he can receive proper care. A cage can actually do much more harm than good. Keeping him for a day or two will only weaken him, you're doing him no good. It would be very sad if a cat got him, but being returned is still his best chance of survival, he needs his parents.