Let him sleep. Hatching is hard work. If he's meant to make it, you'll know within 24 hours. Ensure that he's got a safe space away from the others so that he's not trampled. Did you assist this hatchling?
Vitamin B for the leg thing might be helpful... specifically riboflavin.
I use a product called ChickBooster (TM) which is the most complete vitamin and mineral supplement that I've found for poultry, I just add it to their only available water (although when offered this water beside plain water, they seem to prefer this vitamin water instead). It's saved countless numbers of my birds, both chicks and adults. The feed store also sells a small bottle (with a dropper like you'd find on a bottle of visine) of liquid solution of just riboflavin, without all the extras. For me, and for my time and money, I choose to buy the ChickBooster. It's easier for me, and my arthritic hands, to administer, and eliminates the stress (on my quail especially) of being handled.
Google search offered the below information about poultry and riboflavin deficiencies, though it predominantly focuses on chickens, it applies to ducks and quail as well. Also note that hatchlings can be born with this riboflavin deficiency as the parents cannot put more of something IN the egg that they themselves have a deficiency in). The earlier the bird is supplemented, the faster you can expect healing, delayed treatment can result in permanent disability. If you hatched these from eggs from your own backyard, you may consider supplementing your entire flock.
Google Search Results for Riboflavin Deficiency in Poultry: Riboflavin requirements vary with heredity, growth, environment, age, activity, health, other dietary components and synthesis by host. Poultry species have a requirement between 1.8 and 4 mg per kg (0.45 and 1.8 mg per lb) of diet (NRC 1994). The NRC (1994) requirement for broiler chicks is reported as 3.6 mg per kg (1.6 mg per lb) of feed. However, Ruiz and Harms (1988) suggest that to prevent signs of leg paralysis in broilers fed a corn-soybean diet, the minimum requirement of 4.6 mg riboflavin per kg (2.0 mg per lb) of feed is needed. Olkowski and Classen (1998) likewise suggest a higher than NRC requirement for broilers at a level of 5 mg per kg (2.27 mg per lb). They suggest that the current recommended allowance of 3.6 mg per kg (1.6 mg per lb) is not sufficient for (1.59 mg per lb) modern breeds of broiler chickens. Ruiz and Harms (1989) also reported that 3.5 mg per kg of added riboflavin was needed to optimize the growth of turkey poults fed a corn-soybean meal diet. However, it took additional riboflavin to eliminate signs of deficiency—poor feathering and paralysis of one or both legs. Where sufficient data are available, studies indicate that riboflavin requirements decline with animal maturity and increase for reproductive activity. Chicks receiving diets only partially deficient in riboflavin may recover spontaneously, indicating that the requirement rapidly decreased with age (Scott et al., 1982). However, Deyhim et al.(1992a) reported that 3.6 ppm of dietary riboflavin for growing broilers was satisfactory through four weeks, but that benefits were obtained by exceeding the 3.6 ppm recommendation through eight weeks. Increased dietary fat or protein increases requirements for riboflavin in rats and chickens. It was assumed that high urinary riboflavin excretion during periods of negative nitrogen balance for a number of species was a reflection of impaired riboflavin utilization or retention. However, Turkki and Holtzapple (1982) suggested, in studies with rats, that the effect of protein on riboflavin requirement is related to rate of growth and not to protein intake per se. Microbial synthesis of riboflavin has been shown to occur in the gastrointestinal tract of a number of animal species and thus affects requirements. However, utilization of this endogenously synthesized riboflavin varies from species to species. Within a single species, utilization depends on diet composition and incidence of coprophagy. Carbohydrates such as starch, cellulose, or lactose are absorbed slowly and therefore exposed for longer times to the intestinal bacteria, resulting in an increased riboflavin synthesis. Dextrose, fat or protein as chief dietary constituents decrease intestinal production, thereby increasing dietary riboflavin requirements. Antibiotics, such as tetracycline, penicillin, and streptomycin, reduce the requirements of several animal species for riboflavin or might stimulate microorganisms that synthesize riboflavin. They may inhibit microorganisms in the gut that compete for riboflavin. Alkaline dietary pH appears to increase the level of riboflavin required in chick diets (Donaldson, 1986).
Thank you all so much. I got the chick booster and the riboflavin for the older ducks. All five ducklings are eating and drinking well. The one still has the leg problem, but we will continue to watch him.