Does he have food with water 24/7? If they run out of water or food they will gorge themselves. Are the nares (nostrils) plugged up? They need to be able to wash their heads and clear their nares. https://www.backyardchickens.com/t/750869/raising-and-caring-for-ducklings#post_10611711
Is the water you have provided just sugar water? If so, swap it out to regular water. If you want to add anything, you can add save a chick or the probiotic packets of stuff. Sugar water shouldn't be given all the time, that is just a quick pick me up for a chick/duckling that is needing some extra umph to get going.
