Barley rings are great for putting condition on (although if intolerant may turn him into a nutter !!) - also soya oil or vegetable oil added to feed.
Old Addage of little and often - split meals into 4/5 small meals a day and ad lib forage.
Also - are there any mares in fields near him as...
I have always had horses (3 of) PTS by the gun - lucky enough that my ex husband was always there to hold and to deal with the aftermath of burial as we are lucky enough to have our own land and a council who say if they are not competition horses and they are pets they can be buried !!
I now...