Unfortunately I hate to break it to you, but if you plan on keeping your boy intact you better get used to this. Neutered males can be very aggressive towards intact males. I've seen this plenty of times and yes majority of the time the intact males have zero issues with other dogs, are perfectly behaved, and are still targeted. I would not bring an intact male into a dog park for this very reason, not because the intact males pose an issue (which many people wrongly believe so yes get used to getting blamed by uneducated dog owners who think they know it all), but because they are put in danger from the neutered males. Unless you know all the dogs in the park at that time well enough and know they are good with your boy. Setting up private small group socialization meet ups would be a much better and safer option.
The fact he's getting growled at now and not before has nothing to do with the city you're in but the fact that 6 months ago he was still a baby, now he is a sexually mature intact male.