I have a "foster" pup who became reactive at 5 months due to a total lack of socialization before I got him. After reading a lot about it, I agree with CometDog--gradual exposure and letting the pup LEAD in new situations, according to its inclination, is best. Someone posted about the puppy being tentative near a lake edge, with the water lapping at the shore. They just gave the pup time to explore the beach and edge at its own pace, and that ended with pup enjoying playing in the water. I have been fortunate that my schedules and instincts to let my dogs explore at their own pace worked great with my previous puppies, but honestly, hearing about "socialization" more in the past decade or two, with the arrival of the internet and proliferation of websites, might have led me to over-expose my pup.
Rather than wasting time or energy worrying about it, just back off to a distance that your pup accepts without reaction and let him decide when to move closer. He is so young you have lots of time to let him figure out the world is fun and safe if he is starting to feel a little overwhelmed. And if it is just a bit of growling at distant barking but he can still engage, distract him with a quick training session--a sit, down, stand, look-at-me, whatever you can do in the moment to get his mind off the sound. I've been able to do that with mine just recently--but it took medication b/c his reaction distance is huge, and full-out Cujo if he saw or heard a person/dog (I'm talking a quarter mile or so, now down to about 3-4 yards away). Good luck and let us know how it goes.
Radar, Aussie/BC mix, b. 2/27/2012
Jett, GSD foster pup, b. 7/15/2018