Maybe this will give some inspiration. https://www.backyardchickens.com/t/761308/peking-duckling-with-crooked-neck-help/20
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Wanted to add my 2 cents worth as well. Giving the poultry vitamins is great and you could actually up the ante in his favor by giving him a bit of a human Super B Complex vitamin pill. I agree with Miss Lydia, probably the best thing you could do would be to separate him from the others until you can get him stronger. It doesn't really sound like a Wry Neck going on, but a vitamin deficiency isn't something to be ruled out.
Now the bad news, as least as my experience has shown me...this little one, may always be a special needs duck. The shaking of the egg could have messed with something and it's a miracle it hatched. Am I positive? No I'm not and I would do everything to help get it better. Not sure what you are using for a waterer or what you would do if you took him out of the mix until he is stronger. If you were to go to a chick waterer, the quart size, I would use 1/4 of the Super B Complex pill, crushed and mixed in with the regular poultry vitamins. You will need to find the measure of the poultry vites for this size as well. The Bs are water soluble but some of the vitamins in the regular ones aren't and you don't want to give too many of them.
I will tell you this....I have many special needs birds here. They can be alot of extra work and money, but...they give back unconditional love for your efforts. The best thing is they have no clue they are 'special'. They live their lives doing what they can and are happy. I've been in similar situations hatching my own. I've lost a few and saved many. This is your journey and you have to decide what you want to do. I am sure that Miss Lydia, Amiga, myself and so many others will be here to answer any questions you may run into. I wish you and your little one the very best!