Sounds to me like he is scared of other dogs and as such has resorted to agression , im no dog therapist but everything you have put about his past experiences points to him not having had much luck with his four legged friends , a dog will be agressive on a lead when approched by another dog if its scared because it cant run away so resorts to agression , i wonder if you could find a sympathetic training class who will allow you to attend just to re soscialise him in a controlled enviroment . hope you manage to sort things out with him soon , I couldnt imagine not letting my Labs off lead to run free we must be really lucky that we have never had problems with our dogs or other dogs being aggresive .

__________________