See your vet straight away. It is highly unusual for a horse to not want food. He could be feeling sick and there fore not wanting to eat. Does he have a runny nose or eyes?? Does he seem to be moving around a bit slower?? If yes then see a vet. He may also not find the feed very appetising. Is he still eating plenty of grass??? But don't change the diet until he's seen a vet.

 While you have the vet at your pony's side, you may also ask to have your pony's teeth floated (which means filed, to prevent or cure any sharp, jagged teeth). If his teeth have worn down with any sort of sharp, uneven parts, he'll cut and injure the interior of his mouth & quickly go off his feed.