Does anyone know if there is any hope at all for his survival or should I just put him out of his misery.
It's difficult to tell. If the problem was ammonia toxicity then yes it's something you can correct but your parameters seem to be good so that's not the problem. There's also the possibility that the parasitic infection has spread to his gills and is causing difficulty in his breathing but you put him back in the display so there's really nothing you can do but to wait and see if he can pull through.