I would not force it on him. I would add electrolytes to the water, and apple cider vin., etc like Amiga said above.
I used a couple of chamomile tea bags in warm water in a bucket & dunked my duck's head when she was holding her eye closed (thanks, Amiga!) and then treated her with Vetericyn. I got it at my
TSC & it was easy to apply & not terribly expensive.
It is a good, all around product & should help fight infection, abrasion, and clean the eye, too.
I would think lots of warm baths & limited access to foreign objects - dirt, hay, grass, etc. would be best.
Good luck, poor little ducky.
ETA: You could feed mashed peas, or mashed potato. That might entice him- if he is anything like my ducks, he won't be able to resist.