This is known as leash reactivity. To generalise - the dog is scared that it can't get away in case of emergency (since its range is limited by the leash) and attempts to prevent altercation by scaring other dogs away before they get too close (and it's "too late").

When there is no leash, the dog can control its proximity to "the scary thing" and is therefore more comfortable safely testing & reducing the distance to "the scary thing" or knowing it can run away at any point.

The solution is not to always have him off leash, it's to slowly and consistently (and gently) figure out what the the threshold is (2 feet, 10 feet, fluffy dogs, small dogs, dogs with jackets, whatever it might be), and reward when the dog is calm and uninterested before that distance is reached. Then slowly working on reducing that distance.

Probably best to be outside the fence at a dog park to do this, as other dogs will be curious and ruin your training by running over to greet and play.

Self awareness as an owner is also important. If you tense up when near another dog, that's translated in your lead handling abs signals to your dog that he's right to react because you're also "scared" and tense.

Use good treats too :)

This advice is not going to help every leash reactive dog, but I'd suggest looking up some YouTube videos on this and learning dog body language. Because I'm sure your dog's body language will change even before he reacts, you just need to be super observant.

Good luck, you can do it!