Make sure there is some gravel on the bottom of the tank. Have plants and wood to make the fish feel more secure. Have a picture on the back of the quarantine tank. Keep the water warm and offer live foods. If there's no improvement after 24 hours the fish is probably going to die. But give it a couple of days and it should get better if it's not stressed out.
You could try soaking his food in some garlic juice. This is appetite enhancer. Also there's a product called 'garlic guard' with vitamin C which is the first vitamin to be washed away from prepared foods. So helpful to get them interested in food and also good for vitamins.
I also like to supplement their food with Boyd's Vitachem. It can also be added to the water for uptake thru the skin when the fish isn't eating. Very helpful for your little guy.