Since this muscle is located on a major "movement" part of the hind limb, it needs to heal as much as possible before introducing work. A tear is a rare thing, but if it is indeed torn, just remember that it can be RE-torn. Stretching and icing should be a part of re-conditioning a torn muscle, but reining work should be out of the question for at least 6m (has it been that long since he tore it?). For something this severe, I'd recommend a second opinion (can't hurt) - spend the money you saved on his purchase price on a really good performance vet. A good vet is your first defense against a re-injured horse. Good luck!