I have an old mare with the same problem and currently I feed her a couple of hands (3-4) of shredded corn. But you can also try something else with corn in it. (It is quite sugary, so don't overfeed) we also use beet pulp and unlimited hay with this horse. Checking the teeth might also help as horses can't digest wel if their teeth are bothering them.
You could just ask your vet Clinic if you could drop off a fecal sample to do a parasite check. It will likely be negative, but just in case. We have had a couple of horses that have been on a regular deworming program that have had heavy loads on the fecal test. So we had to do a 5 day wormer. We also have horses that we only worm once a year that have come back with negative fecals.
We don't have square bales only round bales bc we don't have a barn. I can't give him dry feed bc I have to put supplements in his feed and it requires me to wet it down and I do that three times a day. @hannahfrance
What kind of test is that I never heard of it before? Yeah we recently de-wormed all of our horses. We have found bales only bc our horses are out in pasture all the time bc we don't have a barn yet @samiam
I look at maybe either starting or switching to a high fat and amino acid feed. I used Tribute Kalm N Eazy on my gelding when his ribs wear showing. You could also try cool calories(weight supplement).