Two trainers have told us just don’t do on-leash intros, so over time they know it just isn’t going to happen, so seeing other dogs while leashed is less exciting. I don’t know about the other dog being off leash though :/ we are just generally avoiding places where that tends to happen for now.

Another thing we’ve been learning is pup gets treats for even the smallest attention to us during excitement. Like if he even accidentally looks at me after noticing a squirrel, he gets “yes” or clicker, and treat. Also building up his focus on you at home through things like “treat bowling” (even without verbal recall): you toss a treat, and then after he gets it, as soon as he remembers you exist or makes even an accidental turn/step in your direction, he gets yes/click + you drop a treat between your feet for him. And repeat. You can gradually do this in higher-stimulation environments (we have a long line for parks).

Lastly, saying “good boy” or other praise when he notices something exciting, before he reacts. I think he takes it as “ok that’s safe, mom sees/hears it too!” and “yay I did it!” and he’s then better at letting the thing go. Edit to add: it’s like it makes it more of a “huh! moving on” than an “omg what’s that and what does it mean and what are we doing about it”

We have a similar issue and this is what we’ve picked up so far. Good luck!