Wow, ducks sometimes get themselves into such trouble with the strangest things! I am sorry to hear this.
As I wrote in the PM, it may be that the sugar jolt affected him. Right now it occurs to me that there may coincidentally be something else that happened to him or in him during all this. Might you check again for any signs of infection or wound or swelling?
How warm is it where you are?
When Drei was very ill, I could usually get her to eat at least one slug, her favorite protein food. Offering him some protein might help, and keep him drinking as much as you can.
I don't know how possible it might be to get him into a tub or large bucket of clean water to see if he'll perk up a little.
If you want to help expedite the passing of what he's eaten, Storey's Guide to Raising Ducks explains treatment of a duck if it's eaten poisoned food:
confine him to a clean, shady yard or building and immediately provide fresh drinking water with a laxative added - either 1 pint of molasses or 1 pound of Epsom salts per 5 gallons of water.
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It has been overnight already, though. . .
Before you do anything, I'm hoping we hear from you, that he's perked up at least a little bit.