Have you seen worms in their poop? If not, why do you think they have them?
For the limp, rinse off the bottoms of his feet and check for bumblefoot... this will be hard, dark scabs in the pads... or look for cuts, swelling, or hot spots...
Hmmm, maybe separate the drake for a bit and give him some water therapy, see if that helps... just put him in bathtub or something deep enough to just stay and swim for a while...
As for their weight, what are you feeding them? Are they free ranging or penned? Are their keels (breast bone) sticking out?
They roam our backyard during the day and inside there coup at night. I'm feeding them just some basic poultry food and corn. There are no breast bones sticking out. I noticed today that he does this weird thing where he stands up straight and bends his neck almost as if he's trying to stretch?
Corn is not really good for them to have too much of, just a little bit every once in a while as a treat...
Have you had them all since they were ducklings or did you get them already grown? If as ducklings, did you supplement them with niacin or brewers yeast for their deficiency? Could be niacin deficiency, or vitamin deficiency (look up wry neck/stargazing), or it might be neurological... or injury... shine a bright penlight into each of his eyes, check for even and reactive pupils...
I *think* it could be... but I would check his pupils first to rule out injury issues...
Brewers yeast (NOT baking yeast) mixed in their feed is one option necessary for ducklings from hatch to 10 weeks old if they aren't on waterfowl specific feed... most poultry/chick starter/grower feeds don't have enough niacin for ducks...
At this point I would go with a Vitamin B complex though... Amiga knows much more about this than I do... I'll see if I can find one of her info posts...
Well I'm going to Zamzows right now to grab some suplament and some deepened since I should do it anyways 2x a year. Do I need to give him vitamins especially made for ducks?