Sorry to hear about Poptart.
Could you give us some more information about him?
What is he being fed? Has he had any problems prior to his? What's his run/coop area like? What have you done to him so far, in the lines of care for his current condition?
What needs to be addressed first is the anorexia going on. I would highly advise that you start tube feeding him to take care of his current condition, then we can move onto what the underlying cause may be.
To tube feed a duck, you'll need the following.
Parrot baby feeding formula (Kaytee is a good brand). You should be able to get this at your nearest Petco or PetSmart. If not, it can be ordered online.
You'll also need a large 60ml syringe. This should be available at your local feed like TSC, or online.
A scale to weigh him on will also be needed.
Lastly, you'll need a tube to stick down his throat. A red rubber catheter works best, so try a medical store or pharmacy.
Here is some more info on tube feeding,
https://www.backyardchickens.com/th...ng-guide-pictures-under-construction.1064392/
https://www.backyardchickens.com/threads/tube-feeding-ducks-updated-5-7-2020.1211994/