Might I also suggest you check him for accumulated sand in his intestinal tract..
Eating of sand can cause weight loss, depression, decreased performance, diarrhea and colic.
drgarfinkel.com
It is easy and tells you if your horse would benefit from a Sand Clear or similar product be given.
Sand accumulated in the intestinal tract can reduce the nutrients absorbed by the animal...
Removing excess sand also lessens chances of colic and diarrhea happening...
I second looking for a senior feed with as high a fat content as you can find.
Find a manufacturer of feeds who supply your area with ample supply so running out is not a issue...not every manufacturer is sold in every area...learn who is represented in your area...
Senior feeds are made differently to benefit the digestion of aging horses. When fed in appropriate amounts the horse will thrive and gain weight... Many senior feeds can be fed totally in place of forage also when fed in proper amounts.
When you are figuring out how much to feed, each manufacturer has directions on bag back to follow.
If the horse is not chewing well, quids his food {balls it up} he could benefit from being fed a feed with higher fat and fiber content...dense in calories per pound consumed.
Horses need 15,000 calories per day to survive, 25,000 for a horse to maintain in light - moderate work and up to 33,000 calories fed to a horse who needs to truly gain weight.
If your horse is not gaining, instead losing it is because he is not getting enough nutrition fed in to meet the daily basic needs of survival.
Calories also come from forage and forage type foods consumed...
I have given you links to two informative resources so you can figure what your horse is consuming and what and how to improve that so thrive better is what can occur.
https://laminitishelp.org/CalorieFeed.pdf
First though, I would be having the vet come and evaluate the horses condition as he is aged... The dentist or vet if they are good at teeth then to come and makes sure the horse eats and chews well.
I know we are in summer, but...cold weather is soon coming back.
I would suggest strongly you look into offering the horse t/o style blankets and or sheet for added protection from the elements and for the animal to not need to burn off precious calories staying warm and dry...
Blanket sales for winter weight start in July, so best price and selection is soon to begin.
A resource of good information about blankets/sheets and the cut of and weight of, along with denier are all explained in this company brochure seen on their website.
www.sstack.com
And lastly, till the insides of the horse are healed of any damages occurred while thin you will not see much on the outside.
Take pictures of and weight tape measurements to document how the horse is reacting to your feeding approached different.
When you get totally confused about diet and what, how to feed...come ask here.
Our members are knowledgeable and will offer you their opinions on how to proceed....take all of our offered information, do some research of your own and make a plan for going forward you can do and the horse can/will thrive from.
Best of luck.
🐴...