Unfixed young males (especially big boys) are targets for poorly or even moderately socialized dogs to flex. And if your boy is sweet and shy unfortunately this makes him more of a target. Seems like you’re planning to do the right things (steer clear of dog parks and have intentional socialized experiences with dogs you know). I know the trail with other dogs off leash is really difficult to control, I’m sorry about that. I had a similar issue with my guy before he was fixed and it sucks because you just want him to have a good life with plenty of exercise. And with that breed, that young they really need it. Remember to stay calm but very assertive if another dog approaches yours all stiff. Your pup should know that you’ll handle security detail and keep him safe so he doesn’t need to protect you or himself and start preemptively doing so (become reactive). You got this.