Do you have my horse’s twin? 
Mine is 27, with PPID, heaves, get 3+ syringes of meds each day, and tends to not finish his feed if he eats any of it. He eats his hay, and alfalfa cubes. Summer can be hard on him because he eats less hard feed and spends much of his day in the shelter.
He will often eat his hard feed late in the afternoon after being in the shelter, not eating, for the day. Sometimes he will eat early in the morning when it’s still cool. One summer he started eating after we gave it to him in a different colour of bucket. Sometimes he will eat one type of feed so I put the options in separate piles in the bowl and watch what he eats first. Sometimes he eats better if he gets the chance to eat before he gets his meds.
I bring mine in and park a feed bowl under his nose while I’m doing stuff (grooming, Bemer, etc). I put his hard feed and a couple of scoops of alfalfa cubes in it and he eats what he wants. Usually starts with the cubes and often continues with the hard feed once they’re gone. If he stops after the cubes are gone I give him more cubes. Last year he was getting about 2/3 of a small bucket of cubes every day.
I hung a haynet in his shelter last summer. I figured that anything he ate was more than he would otherwise have gotten so if his buddy ate some as well it wasn’t an issue. He didn’t gain back his lost weight, but he didn’tlose any more. I’m starting to do that again this year.
The greatest success came when he was alone in his paddock because I could just leave his feed bowl in the paddock and he would eat it whenever he wanted. He usually did eat everything overnight. I do feel he’s happier with a friend though.
One thing to consider, mine is a lot less likely to eat if he’s working to breathe. Get a stethoscope on your horse and listen to his lungs. If there’s crackles, groans, wheezes, gurgles, etc talk your vet about increasing or changing his meds. At this age long term drug use side effects are of little importance. Quality of life now is the priority. I have a bunch of things I do in addition to meds to help my horse breath better, and I listen to his lungs every day to keep on top of it. Short term use of a more aggressive medication and/or higher doses of regular meds can reduce the inflammation and irritation in the lungs and then allow the horse to maintain on lower doses.