My dog was great with other dogs for 3 years and now suddenly he's not. What happened?

Okay so i'm genuinely confused and kind of stressed about this. My pit bull mix Hector has been pretty social his whole life. He grew up going to the dog park, he's had dog friends, he's always been fine. Then around his third birthday (maybe two months ago?) something just... switched. He started reacting really badly to dogs he doesn't know - lunging, barking, hackles up, the works. Dogs he knows are still totally fine.

Nothing dramatic happened that i can point to. No bad incident at a park, no fight, nothing. One day he was fine and over the course of a few weeks he just wasn't anymore. I've googled this and apparently it can happen around social maturity (2-3 years) but i don't fully understand the mechanism. Is this like a hormone thing? A brain maturity thing? Is it fixable or is this just who he is now?

We've paused the dog park completely and i started working with him on reaction training with some distance. He seems manageable at distance but anything under 20 feet is rough. Should i be talking to a vet or just a trainer? Or both?