Horses can be underweight for a variety of reasons including illness, speed of their metabolism (poor doers), lack of food, heavy worm burden etc. There are serious health implications to the horse if they are underweight, make sure you get his teeth checked and all his vaccinations and worming is up to date before you address his feed. Feeding a diet which is high in digestible fibre and oil will help to promote weight gain without overloading on sugar and starch. For more advice ring the Dengie Feedline on 0845 345 5115