
I'm so sorry you lost him, and sorry I wasn't online last night. Although I doubt there was anything I could have suggested.
Just observations, for anyone who may read this later....
The grit probably didn't hurt him, but normally grit isn't necessary when they aren't outside and foraging for food. Grit is given when they need help with digesting foods like grass, bugs, etc. Its not necessary with chick feed.
Since the mother abandoned the duckling, she probably sensed that something was "off". I've had it happen too, and not just once. Momma ducks are superior at knowing their babies.
The poop pic made me think dehydration, and by the time it gets to that point, its usually too late to fix. Weather could have been a factor, if its gotten chilly there yet. Plus with that many other ducklings, this one could have been a little "slow" and/or gotten squeezed out of the food line, etc. Ducklings can be little piggies and don't seem to care if anyone gets left out, as long as they get their own.
So I feel sure it wasn't anything you did, or didn't do. Congrats on all the other ducklings and I would just suggest keeping an eye on each of them to make sure they are all getting their share of care from momma.