For an aged dog, a shock collar works wonders. Leave it on him all the time, and wear the button fob on a lanyard around your neck whenever you are out and about. If he moves on the horse without command to do so, Yell a firm "Blue(insert own dogs name here) NO", then give him a good zap, followed by another "NO". If you want him to completely leave the horse alone, be proactive. Try to get after him before he makes contact. Since heelers don't stalk, you will have to watch his body language more closely for cues. Usually they will prick their ears and kind of freeze just before making a move. My dogs get used on everything, including horses and goats, so they are taught young to only bump the back cannons with their nose. They only are allowed to become more aggressive if told to do so. This is much easier to teach young dogs though. Some people bristle at the idea of a shock collar, but if used correctly it can be an excellent tool for a intelligent and headstrong breed. What ever method you attempt, be firm and fair, used the dog's name with the command, and be very consistent. Hope this helps!