Working on off and on leash training

Have you tried doing any of this stuff inside where there are fewer distractions? It's easier to learn if there is less sensory noise.

If he takes those treats in other places but not on walks then he's over threshold and unable to learn. Stop taking him for walks. Work inside then the front door. Then move down the sidewalk a little more every week. Look for a happy dog that's sniffing rather than scanning, one that happily takes food, one that will look at you when you call his name. I suspect most dogs that pull and cannot listen are over threshold and need work being outside more than training to stop pulling. When he's happy move a few feet further from home.

What I did with project dog #2, Bucky, was stop when he lunged and waited. It seemed like hours but eventually he would stop orbiting me and be able to look at me. Then I took a step and the world was new again and he repeated the orbiting. I figure when he was done absorbing the environment and could pay attention then he was ready to move another step. This is just reacting to his lunge and only half the picture though. With all my other dogs I easily did 'choose to heel' but not this guy. I found that a 'step and stop' worked better. Give him only a couple inches of slack and wait for eye contact. Now lean forward. If he stays with you then praise him to the skies and give him a treat. If not then wait for him to come back and try again. Bucky adores this game for some reason. I built the number of steps between stops very slowly and bounce the number as the dog is doing. When exiting the house and approaching those places the dog gets excited fewer number of steps before stopping. When going home or crossing the street where it's pretty boring you can do more steps between stops.

I totally agree with working on these skills in a low-distraction environment first - indoors or a yard, if you have access. This way the dog will have a history of being rewarded for the right behavior, and some muscle memory of what they're supposed to do, and be much more likely to fall back to those trained behaviors in a more distracting environment.

The likely reason that you have more success off-leash than on is barrier frustration. The dog knows he's restrained to some degree, and can't perform all of the natural behaviors they want because of it. This can increase anxiety and/or frustration - the main causes of reactivity - which makes the dog reach that point where they're so overwhelmed they don't even notice their favorite treats or toys much more quickly. When a dog is in this state, you can't really train. They're too emotionally amped up. Think of it like trying to teach a kid mathematics on a roller coaster or at a Bieber concert (is he still popular? idk, but hopefully you get my point).

The best you can do in the moment is put distance between your dog and the trigger as best you can (I HAVE resorted to picking up my boy if there's no other way to move him past the other dog, but I also know he will not redirect his frustration into biting me; some dogs will, so keep that in mind). Once you've passed the trigger (the other dog), you can reward for calmer behavior and refocusing on you, but so long as he's fixated you're in a management situation, not a training one.

Each reactive dog is different, and different strategies will work better for some dogs than others. I always recommend Patricia McConnell's Feisty Fido, which is an inexpensive booklet that's really short and comprehensive and specifically about this issue, but you've gotten some good suggestions from other members here too. What's working best for my dog at this stage is rewarding him for choosing to disengage from interesting things. We didn't start with dogs, we started with people or baby carriages or the like, because those are less exciting for him. He'd perk up and watch them, but wouldn't go into barking/lunging/whining fixation. When he decided to look away, look back at us, sniff the ground instead, etc. we'd mark it ("Yes!" or "Good boy!" but you can use whatever), jog a few steps away from the trigger with him, and then reward him with treats. Doing this has made him significantly more capable of breaking focus from other dogs and moving away, which is huge for us. Be patient and consistent, you'll probably have good days and bad when working through these behaviors.

I'll say that the other thing that's made a big difference to us is doing long-line/off-leash walks in wooded/naturalistic areas that are mainly about letting him sniff and explore. I do know that this is a luxury not everyone is going to have access to, but if you have the option it might be worth a try. Doing these regularly has been great stimulation for my boy and has a calming influence. He's much more resilient, responsive, and quicker to recover from outbursts on weeks where he's had at least 2-3 hikes compared to weeks where he just gets neighborhood walks and training/feeding puzzles for mental stimulation.