My 2 ducklings also stopped eating the crumbles after a few weeks. I switched to duck pellets instead, which they liked more, but still didn't gobble it up to my liking. I started making 'duck soup' which is basically a bunch of stuff in a big pan of water that's deep enough for them to fish around in. Dandelion greens chopped, kale leaf whole (also serves as a toy), green peas, berries (whatever treats I have on hand or can grab out of the yard) make up about 30% of it - then I put in some grit, duck pellets, rolled oats which make up the rest - sometimes I add a small sprinkling of crushed meal worms and a dash of ACV. They absolutely love it and they are very healthy and huge so it must be working haha.
Are you considering getting him a new friend? Maybe he's lonely for some feathered companionship... I'm sorry about his sister.
Good luck - keep us posted on his progress.