1) First step, have the vet out to check their teeth for any sharp edges that may be hurting your horse and causing weight loss.
2) Bring some fresh manure to the vet to do a fecal egg check for any worms.
3) Provide free choice hay or grass, unless the grass is very rich, provide a couple pounds of hay for warmth in winter and to make up for low nutrient level in grass in summer.
4) Put horse on high fiber and fat diet, I recommend beet pulp and rice bran, and alfalfa pellets or cubes if the first two are taking awhile to kick in. Make sure you introduce it slowly!
5) Keep feeding grain/hay until the horse gains weight.
If anyone has any other tips feel free to share!