First, IF someone does happen to get nasty about him, have your vet's phone number handy and tell them to talk to your vet since the horse has already been evaluated.
Second, My just turned 27 yo Arab would look exactly like that but for one thing:
I have to give him Omeprazole for his gastric stomach ulcers every January or February for the last nine years
I rescued him 20 years ago and he always has been a horse with very little muscle. Sadly, every year he seems to lose more and every winter his hard keeper stats increase by a few numbers
He also has four missing molars, so along with Triple Crown Senior and some rice bran, I pump him full of alfalfa pellets and timothy pellets (two separate pellets).
Are you sure your horse is eating enough off the round bale? Enough means, between hay and grass, he should get 1.5% to 2% of his desired weight daily in forage.
My Arab does NOT like hay after it gets old. By old I mean anything that has been cut longer than 3 or 4 months. That's why I have to feed the snot face hay pellets. I would feed him soaked cubes but one day he decided he didn't like those either
Meaning, if you can afford even a two week supply of Omeprazole from the vet, try it and see if your horse starts picking up weight.
Also, a good pre-probiotic is prudent to feed to these senior horses.
Since I have had my Arab on "Daily Start" by EquiShine, his bum has been completely dry - even with the new spring grass coming on; I'm not far from you, just an hour north of the AL border in TN.
Lastly, your vet is probably your only vet option but I think I would not take his word that "the horse is just old". In this modern time, 23 is not old.
Get creative about feeding him without going so far over board that you're feeding him too much stuff.
I honestly would try the Omeprazole and see what happens.
I might just take a pic of my Arab so you don't feel so bad. I can see his ribs thru his winter coat and it bothers me every day. Yet, when I'm not looking his old self is out there kicking up his heels and bossing around whomever is behind him in the pecking order