
BAT is well worth looking at, I use it a lot with dog-reactive dogs and it is very useful and effective.
First step for me would be changing his perception of other dogs - a good way to start that is 'open bar/closed bar': basically, when another dog comes into his view, feed him lots of goodies (bar opens) and as soon as the dog goes out of sight, the treats disappear (bar closes). This must be done at a distance greater than that at which he starts to feel worried (as indeed all work on this issue should be). That will start giving him a positive association with other dogs. BAT will teach him to use his body language properly and also that walking away is an option, and a very rewarding one. You want really top-notch treats - meat, cheese, anything really super-duper.
Another technique is to reward him just for glancing at other dogs - I do this a lot in recall training but I have my dog-reactive clients use it too. It makes looking at dogs fun and lets the dog know that it's allowed too (and relaxes them because they are able to keep an eye on the other dog - a lot of dogs can be more worried if they aren't allowed to watch as they can't see the other dog getting closer, or at least worry that they can't).
On walks use escape opportunities whenever you need to - whether it's a change in route, ducking up a driveway or hiding behind a car while a dog passes on the other side of the road. Try to avoid using routes where you will be in a narrow point (such as an alleyway) if you're likely to pass other dogs so you minimise the chances of finding yourself nose-to-nose with another dog. If you do get caught, as GM says, try and keep his attention and just get past quickly and a headcollar can help with that.
Any time at all that he doesn't react to another dog, reward it. And very importantly - if he does react, do NOT tell him off or correct him. When he's reacting to another dog, he will be totally focused on it and anything that happens to him in that moment can be associated with that other dog. So if you tell him off, next time he sees a dog he's going to be thinking "OMG there's a scary dog and OMG I'm going to get told off, I'd better get rid of it quickly..." and so the aggression escalates.
Be aware of your equipment too. Anything that can tighten on him or hurt him (choke chain, half check, no-pull harness) can be associated in that way too and I have personal experience of this happening with a no-pull harness - my first dog-reactive dog couldn't wear a harness of any kind for nearly a year as she'd associated the pain of it tightening when she lunged at dogs with those dogs, so any harness made her aggression considerably worse.