Age is very important as it defines the stages of development.
You should have a working relationship with a Avian Vet since you have at least two older Ringneck parents. A Certified Avian Vet visit is highly recommended.
If the issue is not age development based it can be diet based or other medical issues, which supports setting-up an appointment for tomorrow morning.
Eclectus, CAG, BH Pionus, Maximilian’s Pionus, Quakers, Indian Ringnecks, Green Cheeked Conures, Black Capped Conures, Cockatiels, Lovebirds, Budgies, Canaries, Diamond Doves, Zebra Finches, Society F
Senegal Parrot named "Kane"; Yellow-Sided Green Cheek Conure named "Bowie"; Blue Quaker Parrot named "Lita Ford"; Cockatiel named "Duff"; 8 American/English Budgie Hybrids; Ringneck Dove named "Dylan"
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?