They can worry us. Too bad you don't have chicks small enough to put one with him, that helps sometimes for them to see the chicks eat. maybe try a mash with a very small amount of crumble and a tiny bit of ACV added to the warm water then mixed into the feed. The reason I say small amount you won't be wasting much if he still won't eat. I make a mash of an evening for my flock using ACV and warm water let it sit over night. They all love it. It's not soupy but moist. It would be nice if you could rig up a way for him to be with the other ducklings and chicks with out them being able to get to him. That way having interaction with the others may just spur him to eat. especially if he sees them eating their feed.