My loving staffy is aggressive to other dogs

Hi, my staffy, George, is 6 years old, I got him as a rescue dog when he was 6 months old and he has always been aggressive towards other dogs. I’ve had him at dog training when he was young, and he was great, he wouldn’t socialise with the other dogs but they didn’t seem to bother him. However, now he is so aggressive to any dog he passes. I can never enjoy a walk with him if there are dogs around, he just flips and literally sees red, there is no getting through to him, once he is fixated i cannot distract him, i’ve tried food, talking, shouting, tapping him, tapping him a little harder… he just goes into a rage. How can I get this out of him? He is so loving and gentle towards people so it’s a shame as people label him as a bad dog due to his reaction of dogs. I really need help. Thanks