Hi Everyone :)
Thank you so much for your advice, words of encouragement and support. Sadly, little duckling didn't make it. I feel pretty sure that the trauma that crushed his egg caused the problem. That kind of pressure had to do something to him. While he had yolk sac to absorb, he was progressing but once those "built in" nutrients were gone, he had no interest in food or water and grew weaker. It probably would have been painful for him to ingest food or water. And I suspect he may have had internal bleeding. He was definitely deteriorating and so my husband didn't want him to suffer. It's so hard to make that decision.
Like Duckmama, I had a little one from another clutch hatch early and with her yolk sac which was still a considerable size. I put her in the incubator w/ a damp cloth around her and I was astonished at how quickly she made progress. She, too, is the runt of the litter but such a busy little thing---smart, healthy and keeping up w/ her sibs. I'm glad I had that good experience with her. I believe every experience--happy and sad alike---helps me to be a better duck mom for the ducklings that follow.
Thanks Wynette for following up with me. So nice to have a friend out there :)