He doesn't seem to mind the boots and is still eating well so we'll see how the vitamin B does. I only gave him 1/4 ml into a quart of water because I was unsure of the niacinamide, but I'll give him 1 ml tomorrow. It seems like swimming might be helping him too. It gives him a chance to use those muscles without having to support his full weight. 
I bought these  eggs, 12 of them, with 11 making it to hatch. One never developed. I got them because a few years back I was forced to incubate 7 eggs from a nest next to our porch that the mother had abandoned. My dogs made her abandon it so I felt I had an obligation to take care of them. They turned out fine and I released them into a large neighborhood pond with (probably) their siblings from wherever their momma made her second nest. Or, that's what I believed because there were 6 adolescents of the same size as my guys. For the last several years local ducks have stayed for the winter at our local ponds. 
But I am teaching them to get bugs from the water (meal worms and cricket parts) , and to eat grasses and bugs from the back yard. They're going into a movable pen this weekend. I haven't been handling them because I don't want them to lose their fear of big mammals and my dogs are helping with that. They obviously know the food comes from me but they'll have lots of water plants and minnows at the pond. Plus, they will get treats from the local children, though I'll be educating them not to feed them bread. 
Of course I would never release limpy in this condition. I don't even know if he would make it as a yard duck if he doesn't improve. So I'll be doing everything I can to help him with this. I really appreciate the help.