How can i get my pony to loose weight?

to get me boy to loose weight he was/is one rationed hay when in the stable (also putting an extra haynet over the one which is full will make the holes smaller and therefore take longer to eat), horse walker, lots of hacking - hill work is especially good, in the arena lots of trotting, he was either turned out in the starvation paddock or in a normal paddock with a grazing muzzle (he hated it but its for his own good, so he had to put up with it)