I respect everyone who has commented but I have to disagree on a few certain points. Please do not wait and see if he grows out of it. He will not. Dog reactivity only ever gets worse, not better, without intervention. It’s important to eliminate this behavior asap or it will solidify into a damning habit that will be harder and harder to get rid of as time goes on.
(The exception to this is if it is rutting like Zande mentioned, but you can’t know for sure if that’s all it is)
If he’s attacking dogs at the dog park, then I second those that suggested to (unfortunately, I know) stay away from the dog park.
But leash reactivity is honestly one of the most common and most fixable issues that pet dogs have. I’m not sure what you’re walking him on, but if it’s a harness, I strongly suggest you look into a martingale or prong collar (and then, of course, do lots of homework on how to use these tools properly - there are tons of fantastic videos on YouTube, which is a great place to start). If you control the head you control the dog, and solid leash manners are going to be key in eliminating leash reactivity. Do not try to ‘redirect’ the dog with a treat if he starts to growl, vibrate, lunge, etc. because a) chances are he will be too aroused to even notice the food and b) if he does notice it, he will interpret the food as a reward for the bad behavior of growling, lunging etc. and the problem will escalate. This is, unfortunately, a huge misconception in the dog training world. Food in response to shitty behavior will always, always be interpreted by the dog as a reward for said behavior, not as a deterrent.
Of course treats will come into play when you’re doing leash work. Start with zero distractions in your driveway or another private area and reward him for heeling and keeping his focus on you. Once you’ve done tons of repetitions then start adding in distractions like other dogs. Again, don’t respond to any reactive behavior with treats, but do reward him for ignoring distractions and keeping his focus on you.
I would also strongly suggest investing in a muzzle. Safety is #1 priority for you, your dog and others, and no matter the reason for the behavior, muzzles create a safety net that will allow you peace of mind when out and about with your dog. Baskervilles are the most popular and allow for eating, drinking and panting. If you size it right, your dog will barely even notice it’s there.
Edit: Also, there’s nothing wrong with not neutering him yet. You’re really not supposed to fix dogs until they reach sexual maturity, and that usually comes between 1-2 years of age (For bitches it’s after their first heat). I mean, you can, of course! But you don’t have to feel pressured into it. Like others have said, chances are it won’t fix the problem anyway