How is he doing this morning?
Judging from the picture, he doesn't look like he's feeling well at all, so I think you've done well bringing him inside. Sick birds tend to have lowered body temperature, so if he isn't, move him to an area that's fairly warm, of course, with feed and water provided.
To help prevent the onset of dehydration, you can drip sugar water along his bill every so often, or offer him soupy feed, and see if he'll take to that. Doing that will help keep food/water inside him, but if he continues not to eat/drink, you'll need to tube-feed, as it's almost impossible to get his daily amount of feed/water just by dripping along his bill. Here is some info on tube feeding :
https://www.backyardchickens.com/threads/tube-feeding-ducks-updated-5-7-2020.1211994/
Is he pooping much, if yes, could you post a picture? What is he being fed? Has he or any of your ducks had problems in the past? If you gently palpate his chest, does it feel hard, watery, like a beanbag? Does his breath smell ok?