Magnesium for Horses | Natural Health for Equines
gives a great deal of info.
MagnesiumOxide should be available at a feedmill. Remission or Quiessence, if you can't get the MagOx. Best would be MagRestore(google it, I don't have the link).
You'll have to fill up his storage, loose stools signal "full"
Flaxseed, whole, 2-4oz/day. As said, for the Omegas, hoof, hair, skin, digestion, help eliminate sand.
Vit E...I get the gel capsules for people, to arrive at 2000IU daily.
All that mixed into a handful grass or alfalfa pellets, soaked. And a general vit/min supplement.
Start him slow with the slowfeeder, havd some hay on the ground, and the net full, so he doesn't get frustrated and starts pawing at it or ripping it apart. Pull some hay through tge holes, so he gets the idea. Mine prefer the nets over hay on the ground now.