I have dealt with a type of lameness similar to this (although its hard to tell obviously from just reading a forum).
It have seen similar injuries diagnosed everything from arthritis in the joint, tendon injury, to a hock or knee injury, to full blown EPM. That's why I asked about the toes dragging. Does he seem coordinated still, like can he move freely, but just is stiff? Can he back up okay? Can he still cross his back legs if you ask him to move over?
It could really be a number of things. He could be sore from too much work, could have twisted wrong in the pasture, or bumped his leg on something, gotten kicked, or fallen down. Or he could have some other disorder. Hopefully it is just a "twisted ankle" of sorts and will get better soon with some rest and frequent icing. Unfortunately you won't know until the vet comes.