Need help with Muscovy duckling issue

Hello everyone,
I have a runt Muscovy duckling. He/she should be around five weeks old. I will stick to calling it a him for sanity's sake. The problem is he flips over when trying to move in any direction. He mostly sits on his elbows with his feet stretched out in front and scoots backwards. He will then try to run forward and fall to the side. He is way underdeveloped, the size and look of a 2 week old. I've resorted to keeping him inside in a giant plastic tub. He sleeps on a heating pad (on low) between his sock (filled with rice) siblings. He sleeps almost all day. I keep electrolyte water (started the vitamin addition last week) with cracked corn and floating catfish food available at all times. He takes regular baths about every two hours (of course I put him in the water and dry him off). He doesn't swim around very much, mostly just sits there. He has regular number 2s. I am asking for any help, please. I have researched every possible idea. I am attaching a few pictures for reference to size. In the last photo, he is the farthest on the right.

Are you feeding him actual poultry grower/starter? If not that could certainly be a part of it. It seems like this a neurological issue. I suspect wry neck or something similar, which is caused by dietary and nutritional deficiencies. It can actually start in the egg, but proper diet after hatching is really important to prevent it. Does he keep his head tilted to the side like that most of the time? Any running in circles? I would switch him right away to an actual poultry starter feed if he's not on one and I would also put him on a vitamin supplement. I recommend Poultry Nutri-Drench, which you can get at any feed store. Start giving him one or two drops orally a couple times a day. Make sure not to force it though, or he could aspirate it. Open his beak and keep it level, gently squirt a drop or two in, and let him tip his head back himself to swallow it. If this is wry neck this will clear it up, although it can take some time. If it's not wry neck, it won't hurt, and with him being underdeveloped it should help with that anyway.

He only tilts his head while he is sleeping, and he can't really run in any direction. I will stop by the feed store today and get him both the feed and nutrition supplement. Thank you so much for your help! I will try to upload a video to YouTube so it's possible to see him in action.

I have sad news. He started breathing funny this morning after I scheduled a visit to the vet (he treats ducks). The vet diagnosed him with Marek's disease. There was no hope. We put him down, so he didn't have to suffocate.

Wow, sorry about your loss. Merek's is extremely unusual in ducks. But, nonetheless, you might want to take precautions so that your other birds don't get it. Hopefully, your other ducks will be fine, which they probably will because, like I said, it's rare in ducks.