You must do what you vet suggests then and I wish you luck with your horse - though I'm not quite sure why you've asked us for advice when you were always going to do what the vet suggested?
I would personally have serious doubts about any vet that said I should feed my horses a supplement on its own for weight gain but its your horse so the choice is yours
Quite apart from anything else its not a financially sound idea since you can get all of the things that are in Amplify plus other essentials for less money in a complete pelleted feed that has more nutritional value
I was just curious of amplify vs rice bran. Those are what he suggested and I just wanted opinions on the two.