Try a smart pak, if you can afford it(since it seems pretty expensive now). You can take the smartpak wizard test to see what supplements would be best to help him/her gain weight!
feed him in between every meal
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.
Vegetable oil, apple cider vinegar, and Karo Syrup are all good weight builders. Just add a little to his feed everyday! (and I'm personally using veggie oil and seems to be helping!)
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.
Cool calories Supplement works great
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 would maybe get a fecal test done on him. Even if he has been de-wormed recently he might have some resistant parasites.
And lots of good quality hay. A lot of people like to feed grass hay, but I prefer alfalfa. My horses do really well on it!
Put some oil on his grain like an omega 3,6,9 oil
That's what I do. I feed him twice a day everyday and put weight gainer in his food and that still not working so I don't know what else to do @arabcrazyy
I'll try those with my gelding and see how they work out @eclipse_98
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).