Try feeding down a tube directly on top or in front of him, do it several times a day as flounders are well known for starving to death, mainly because improper foods are offered but if you've got worms that it was confirmed were being eaten at the shop, it's a good start. Try blood worm also, both type of worm at same time perhaps.
Easy to feed using a rigid pipe like an inch diameter, I've fed many slow feeders like banjo catfish with this method
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