I have a sick pony

and we can't figure out what's wrong. We've had the vet over, and he hoped to get bloods and worm counts etc back yesterday, but that didn't happen so we have to wait til Monday.
Poor pony is just losing weight before my eyes.
He's spent the day sunbathing, wrapped up warm, eating extra small feeds of high protein 'build up' feed, and generally feeling sorry for himself.
Poor boy.