I've been doing some more research about hock sores. I've learned that one thing I've been doing is probably counterproductive - I've been putting various ointments or Vasaline on the sore, but it seems that what is needed is for the wound to form a protective callus and the ointments inhibit that by softening the skin - apparently crusty is the way to go. I may put a piece of duct tape or a surgical bandage over it to give it a chance to crust over.
I'm also going to put more soaked pellets in his stall today - the deeper bedding will help (although it makes the stall harder to clean) - with our bedding it's the hard, not yet broken down pellets that may be causing the problem.
And I've found an interesting product on-line - this woman apparently was dissatisfied with available products (most of the reviews I've seen for the hock boots that are widely available say that they don't fit well, aren't durable and don't stay on) and designed her own hock boots - these look pretty well-designed and durable to me. I'm going to try to call her Monday and see if I can get a pair.
In all my years with horses, I've never had one with hock sores - our Lily used to rub the hair off the front of her front pasterns from lying down on the hard ground, but she never got sores.