Hiking with 6 month old puppy with slight fear-aggression?

I agree with getting tense. Pups, generally, sense every emotion you feel and feed off of them. To me though, it sounds as though he is aware of the “threat” the other dog may cause but also there lack there of when on leash. Have you ever tried using a chest harness? The cushy padding is kind of like a sweater or a “hug”. It can give pups who are feeling a little agitated or stressed around other dogs a sense of security. Just like the snuggy blankets or thunder shirt (same type of idea) If he remains calmer and you remain calmer then he will learn that other dogs are not always going to be a threat.

Most pups do best the more you socialize them frequently so they lose their fear of other dogs. Since you are aware of the issue everytime you encounter another dog you tense up and that tension goes right down the leash to your dog telling them to be on guard, so your behavior needs to change to be more relaxed. Talk to your dog, tell them good boy or girl, allow your leash to go slack when being approached. Also I would stay away from off-leash areas until your dog is better prepared for these encounters

I would also add that since you are aware of a potential aggression, you should consider a muzzle. This does not mean he is bad or mean but protects you and any potential animals being injured should he react negatively. I'd work on a lot of focus work and redirection both to keep you moving freely as well as him. I would also warn others that your dog is not comfortable with other dogs should an off leash dog start to approach and then move in another direction. Having a fear reactive dog myself, I avoid putting him in too many situations where he will feel reactive, however when he has to be I make others aware and keep him " working" so as to essentially work through the distraction. Be aware of both his and your own body, he might give you many warning signs that he is not comfortable before snapping, it is your job to read those and remove him or redirect his attention before it happens.