When a horse is underweight it coud be from many reasons, stress, not proper feeding, poor pasture, worms ect. A really easy way to put weight on a horse is to ad karo syrup to their feed you would be amazed how well it puts weight on them. Put enough in their to lightly cover the feed. Also make sure you feed ALOT of hay because that is what horses pretty much get there protien and weight from make sure it is a good quality hay also not just a filler
Having the horses teeth floated. A must to check your horses teeth for sharp points. There are folks who float teeth (rasp) reasonably if you look into it. Make sure they have a good reputation. Some vets dont always want to do the job. I have someone who is awsome in S. Ca.
If the horses teeth are good he won't even charge you. The weight will change within a week.
In addition to a regular worming schedule and teeth floating, adding vegetable oil to the feed ration a good way to put on weight. Depending on how much feed you are mixing with it you can work your way up to a half a cup per feeding. This is a great way to add the fat but not the starch which makes horses excitable.