Howdy and welcome to the forum
I wouldn't use duct tape because there is no give to it, especially for it to be so close to a joint. You might consider trying a bandage with vet wrap around the outside over the antibiotic ointment, but you always have to be very careful not to wrap it too tight where it would cut off circulation. I don't know much about hock boots so I can't say much about them.
How soft is the wood chip bedding? Is there any chance that it is abrasive enough to keep the sores inflamed and un-healing?
Also, what kind of rocky terrain are you talking about? Is is more like a desert setting where the ground is nothing but rocks or is it mostly grass with just some rocks mixed in? Only reason I ask is I am wondering if going through his paddock and removing rocks would be possible.