We have gone through most of the same issues with our aging herd. Lost an old thoroughbred mare last fall at 34. She had a stiff rear ankle, but looked awfully good for her age.
She was pretty finicky and kind of tough to keep weight on the last few winters so I will add something we did that I haven't seen mentioned yet. Found a "weight builder" supplement and made a thin slurry out of it and got her to take it with a large syringe. If we tried to add anything at all to the regular grain feeding she just turned her nose up at it so kind of had to "force" this on her, but no worse than administering medicine. As you have mentioned watch for signs of founder and gastric issues if you try this.
That last call to the vet is pretty tough. She had been doing real well, but went down in her stall and did not get up again, assumed heart failure but who knows. Kind of a tough task getting an 1100 pound animal out a 4 ft. wide door.
Good luck with your guy,