It is very wrong to use prong if your dog exposes agression, it works as a bite on the neck he only expects. Just don't look in these dogs direction yourself and walk slowly and confidently away from them if you are on your way somewhere. But you can start training him doggy etiquette instead of simply avoiding the trouble. In a friendly way two unfamiliar dogs never come face-to-face ( only friends do that, because they are not afraid of each other), normally respect is shown by walking head down and tail up on each other side and sniffing each other ramp. The encounter may end with this ritual, they may try playing, and, if they fight - it would be some non-related to their meeting reason. You start your training by changing your pace to slower and asking your dog to heel when you walk pass that dog. Pay attention to that dog posture. If he stands in one spot motionlessly, head up, all four legs straight, seemingly holds his breath - it means he is ready to lunge doesn't matter what, don't pass him closer than 15 feet. But, if you see his back legs bent, he breathes heavily, even if he pulls on the lead and barks - he is not dangerous, but self-protective. Walk with your dog heeling, and only you pass him couple of feet - loosen your leash, let your dog make his choice. If he chooses to be with you - walk away, but if he chooses to come closer - start making a semi-circle around that dog and his owner leaving him a longer leash. Longer leash is important, it tells your dog that you are not terribly keen on following him. Saying "Gentle" not more than twice at that moment in a calming voice is important too. Training your dog with unknown females prior to meeting unknown or protective males would make it more understandable for your dog. Socializing your young male requires knowledge about dog rituals, in many cases it is the lead what makes them fighting.