Is he/she a thoroughbred?? If so I would call I vet because worms are very common in loosing weight. Also when I talked to a vet about the same thing they said to mix a little bit of colonial oil with rolled oats!!
Contact a vet, could be something else than the food. i would contact a vet first and get advice from them, then eventually give more hay and some feed with high fat levels or add some vegetable oil to his/hers feed. has helped mine alot in the winter, when she usually goes down in weight