I donated some port cichlids to a local cichlid dealer several years ago, and when I went back a year later they still had one left in a mixed community tank. He was in the same condition. I felt guilty and brought him back home. I wormed him for roundworms and tapeworms, put him on good food, and the sunken in look never went away. He lived for a few more years but he was never a chunky, well-rounded acara again.
If you do bring him home, I would suggest worming him. I do this with all the wild cichlids I bring in. I do a levamisole treatment (24 hours), then a major water change, then let him rest a week with good food and clean water. You can do two or three 24-hour levamisole treatments, one to two weeks apart. This takes care of roundworms like capillaria and other parasitic nematodes.
After resting him for another week or two, you can treat him with praziquantel for flatworms like tapeworm. A single treatment of this is usually adequate. See if he passes any tapeworms. You can also add epsom salt as a laxative 12 - 24 hours after the prazi treatment to help him pass any dead worms that may still be in his body.
Other than these two treatments there really isn't much I'd recommend. There is always metronidazole for intestinal flagellates like hexamita, but unless he actually shows signs of those (white, stringy feces, lack of appetite, etc.) it's best not to treat the fish for it. If he eats well just feed him a good, varied diet and keep his water quality high, and that's your best chance of turning him around. I have a sneaking suspicion that he'll always look a bit sunken, though.
