[Judi W]

I'd correct him off, not just pull him off and until he learns better manners I'd make him drag his leash so I could interupt the very thought with a correction. Don't wait until you see him riding another dog, correct him for thinking about it. If you don't think it would be safe to grab his collar, then you have more things to worry about than his mounting behavior. You really need to explain this to him in a way he understands or he is going to cause a fight at some point.