The fish might have injured itself when it landed on the ground. Now it is trying to recover and is feeling unwell. It might never recover completely if it did some internal damage.
I would monitor the fish and keep doing big regular water changes.
Try him on different foods, even if you go outside and find aphids on a rose bush or catch small flies or mosquitoes, or mozzie larvae. Just make sure they are free of chemicals. See if he eats those items.
If you can afford some frozen food from a pet shop, or even some live brineshrimp, it might help bring his appetite back. If not, you can try offering him bits of raw or cooked prawn or fish. You can buy small packets of prawn/ shrimp from fishing stores in the bait freezer, or from supermarkets. You only need a couple and keep them in the freezer.
Take one prawn out and defrost it. Remove the head, shell and gut (thin black tube in body) and throw these bits away. Use a pair of scissors to cut the remaining prawn into little pieces and offer 1 or 2 bits at a time. Let him eat as much as he likes then stop feeding and remove uneaten food so it doesn't cause water quality issues.
Offer dry food and frozen or live food several times each each day and just see if he eats more.