It's not the same dog. Same horse, different dogs. He's been attacked before so is now understandably frightened.
Joji - I've heard about the turning the horse round thing, but he gets impossible to stop. He just goes. Not flat out bolting fortunately, but I can't stop him enough to get him to stand still. Also one of his other ridings did stop him recently in this situation and he reared up and became much more dangerous.
Last time after I came off the dog kept on chasing him, but it ran off before I could catch it or find the owner. grr.
I think I'll just stick to my plan of only riding him at home. The dog in the home paddock thing was an absolute fluke and I don't expect that to happen again, so at least I feel safe there. I love the horse loads, so I will not stop riding him!