Yeah, we need a lot more information here...Depending on his age he may or may not be ready to Fledge yet, or fly, but if he's "not using his legs" at all, which they should be able to do from an extremely young age, not long after hatching, then that's a problem...
Did you have bedding of some type in the bottom of the nest-box? As SilverSage mentioned, he may be "Splay Legged"...Could you post a photo of him?