If the joey is nursing and mom is caring for him - you do not need to feed him anything.
Actually feeding a 2 week OOP joey will reduce the amount of milk he will be able to consume from mom - and if his nursing is reduced, her milk supply will also reduce and he will not get all of the nutrients he needs that are contained in the milk.
At about 5 or 6 weeks OOP the joey will follow the parents to the food dish and begin eating the same foods you offer the parents. Just offer a little more of all the foods you feed each night when you begin seeing the joeys at the food dish.