If this turns out to be the problem, there is a surgical procedure that could allow you to get him sound enough to finish for slaughter. I doubt it makes financial sense though. My Vet would probably do it pretty cheap, but I expect most Vets would need to charge more than what the animal is worth.
I saw this in the Merck Veterinary Manual:
There is no successful medical treatment. Because spastic paresis is heritable, affected animals (especially breeding bulls) should be eliminated from the herd. Palliative surgical treatment may be attempted, although ethical issues should be considered when breeding stock is involved. The procedures, usually performed on calves, include complete tenotomy of the gastrocnemius tendon, which results in a dropped hock; complete tibial neurectomy, which results in sufficient relief to permit a steer to be finished for slaughter; and partial tenectomy of the two insertions of the gastrocnemius muscle and the calcanean tendon sheath, which overcomes the problem of the dropped hock.