Bad doggie behavior....how can I break this habit?

I rescued a 3 year old mini schnauzer and I LOVE him. I have only had him 2 months but have noticed that he curls his lips and lunges at other dogs.
It only seems to happen when I'm near the other dog. While on the leash he doesn't go for other dogs that are walking by and is good at the vet. Does anyone have any suggestions on how I can break this habit?
I know he was not properly socialized as a puppy but Im not sure how to fix it now.
Thanks in advance.


Hopeful to fix behavior!