Meeting another dog

My dog is a 2 year old rescue. He’s very reactive but we have been working hard on his training. He loves the dog park, and goes on adventure walks with a group of dogs each week. But on leash is a different story. It was never a huge issue, but recently has becoming worse. Things will be going good, tails wagging, they greet and have a sniff and then 10 seconds later my dog switches and attacks and we’re all caught off guard. And then sometimes he’s completely fine! It’s unpredictable and there are no negative signs leading up to when he’s going to switch. The leash is loose and sometimes I talk him through it which I find helps. Any advice on how to approach this? Thank you!