Sadly i am not an expert,wish i was.
Have you checked his foot for bumblefoot,could he have suffered a sprain/fracture in his leg,is there any swelling in leg? Did you notice any improvement in walking when he was on metacam? Green poop is from not eating enough and sleepiness i would attribute to that also. Any damage to his beak? Any crop issues,referring to an impacted crop,if he is not eating i would think his crop would be empty. Does he have access to grit being inside home? You could try aspirin for leg(great for pain/swelling)81mg baby aspirin or reg 325mg,dose is 25mg per lb of body weight,crush and place in water(my preference)or sprinkle over feed. Try adding warm water to feed,so it resembles oatmeal he may like it. Falling backwards is probably due to him being unsteady on his feet from leg issue also could be from weakness. You mentioned that there was a storm,was he caught in it,could he have suffered some injury,did his limp start after storm hit? It is possible for any animal to suffer a stroke,any issues with wing on same side as leg issue? If his comb is big and red he has good circulation,but opening and closing beak means something,possible respiratory issues. Look inside his beak,you are looking for anything unusual.
You may say you're not an expert but you give solid advice! These symptoms , loss of balance or use of legs, one eye supposed blind, and behaves as tho he's eating but his aim his off and is not actually eating, sorry, reminds me of the symptoms of Marek's. Some of my Marek's victims have also had the gasping aside from normal breathing. Usually all this means , maybe, is the Virus is affecting the nerves , and the eye.
I would also be giving some thought as to the stress of tube feeding and tube feeding prolonging the inevitable. It's so sad. I'm not an expert either, but I am offering you a possible explanation.