Sounds like a tough situation to be in, Im sorry you were left high and dry by the rescue. They should have been giving you advice and management help from day one.
I suspect your dog goes nuts at other dogs in an attempt to keep them away through fear more than aggression. The problem is every time he's put in a situation where he feels he has to behave this way the more he gets to practice it and the harder it will be for you to build his confidence again. At the beginning he probably displayed subtle body language letting his person know he was feeling uncomfortable and wanted more space (think of this as whispering) but where this was ignored or missed he's had to revert to really obvious body language to be heard (yelling). Definitely don't growl at him for showing you how he feels, it's the quickest way to have a dog which goes straight for a bite.
Re a behaviourist my suggestion is to find someone who practices force free training. I found a trainer in Melbourne who is part of an awesome guild of force free trainers - fingers crossed they're somewhere near you. http://petprofessionalguild.com/Sys/PublicProfile/28664856
There's also a great facebook page called Reactive dogs UK which is amazing for advice on reactivity. https://www.facebook.com/groups/1633448230248202/
This is also a great page on dealing with fearful dogs. It has great resources for learning how to counter condition, operant conditioning and de-sensitisation to help with different degrees of fearfulness or anxiety to help them feel safe and learn. http://fearfuldogs.com/
I wish you luck, thanks for caring enough to seek help.