What are you feeding him now and how much? What kind of hay does he get? Go to your barn and write down what's on the feed tag. If you don't know what kind of hay it is, ASK the barn manager. Also ask how much he's getting. Do you give him any supplements? When was the last time his teeth were floated? When was the last time you dewormed him and with what products?

Giving a horse more feed isn't always the answer to a weight problem. If a horse has a reason for weight loss (other than just not enough food), then you'll end up pouring a LOT of food in him with little results.