No, you can't give him only mealworms. Only a couple as a treat
You can use chick feed if you don't find anything else. But you have to be absolutely sure that it's not medicated, it is toxic for ducks.
He needs a starter, finishers are for grown ducklings, which need less proteins; probably finisher pellets/crumbles have also a bigger size than starter feed
If the feed can't arrive at your home tomorrow morning you should search in a store near your home today. I'm worried that may be too late to wait too much...