First I would call the vet to know if your horse has parasites and to have a blood test.
Some time ago, my horse also had trouble gaining weight, even if he was apparently healthy, blood tests told us he had some anemia. My vet gave him a supplement and in a few weeks my horse was perfect!
I agree with the girls. You can:
- give the horse more pellets
- add corn, soya in his ration
- give him more hay or a more nutritious hay
- check he has no health issues (stomach, teeth...)
- add Isio 4 oil in his ration of pellets
Did you check that your horse doesn't have any illness or dental issue?
If he doesn't, forage may be ta good way to gain weight, but it’s not a calorie-dense food, and there’s a limit to how much a horse will eat in a day. If your horse has been consuming all of the forage he wants, and he still is not gaining weight after several weeks, it’s time to add some more calories to the ration! :)
You can give to him more pellets to help him gain weight ! Maybe freely hay as well :)