So sorry to hear your story, your Mom have done a wonderful thing taking in this dog, chipping and neutering him. Everything a good dog owner should be doing. The no jump collar could work - if you can find it in that size. I had another suggestion for you, a electric fence. Run a wire or white tape either around the top of the horse pasture or the top of the fence where you want to keep the dog - I don't know if he cleared the fence, and at that size very possible, but if he had to make contact with the top of that electric fence he might not want to try it again. We have been keeping our horses in a pasture with only the white tape electric fence for over a year now and it's been working great - the dogs also know not to make contact with that white tape.
Best of Luck
