I agree with colliepoodle. You need to catch his attention long before he's even aware a dog is near by. It's not easy and it means you have to be on your toes all of the time. You need to be three steps ahead of him!!
I'm sure your behaviourist gave you all the right advice, the crucial element is timing and stepping in soon enough to prevent him reaching 10 on the anger scale - when there will be no return!
I always like to think like the dog (or how I imagine the dog thinks
) .... ''dogs equal treats, equals I like dogs''. It's worked for me with several dogs, but you have to be all over it and persevere, it's not a problem that can be fixed overnight as he could be harbouring deep rooted fears dating back x