Most of the time the ones you help will be weak for a few days. Adding sugar to the water or an electrolyte supplement will help give him energy, but he will probably struggle for the first few days. The cord may stay on for a while, but it is not really anything to worry about. When it does come off or he is no longer attatched to the egg, make sure the he has fluffed up a little before introducing him to the others. If part of the cord stays attatched, do not try to take it off as it is connected to things inside the duckling. Let it (the cord) dry completely and over the course of a week, slowly trim it off. If other ducklings peck at it a lot, you may need to move him somewhere by himself. But otherwise, just let him gain some strength and good luck with the rest of your hatch.