I agree with Nutty, it could be a hind leg that is lame. However, it also could be a stone bruise, a pulled muscle, a strained tendon, a twisted joint, or any number of other things. It could also be an abcess in the hoof or his back could be out of place. The only way to really know is to take him to the vet and have him checked out. If that is not an option, then I would suggest walks as often as possible and try not to let him trot or lope on hard or rocky ground. Just take it easy for a couple or 3 weeks and go from there.