Thank you for your input. We pretty much do the things you suggested when walking him. When we are on an open beach or lake, etc., we will give him a longer lead to explore providing there are no dogs on the beach. This last trip while at a large lake, we could take him to a beach everyday where there were no other dogs and he was able to run free, swim, retrieve and explore. If we are in a confined area such as an RV Park where the walking areas are somewhat narrow, he is always on a short lead and we try our best to keep him moving and avoid letting him have eye contact with the other dogs. If the area is too narrow and eye contact is nearly impossible, that's when we put him in a sit and have him look at us. If he starts to turn his head toward the other dog, we tell him to watch us. I'm always telling him "It's OK." Trying to let him know that he is alright and not in any danger. This has helped a lot. As far as what we do if he lunges and goes after another dog, we correct him with a sharp tug of the lead if it there is no contact, but if he's close enough to reach the other dog and goes for it, we pull him back immediately and either put him down on his side or put him in a sit, telling him No. When he shows any sign of relaxing, we tell him "It's Ok" again. But we do not let him get by with any aggressive action without being quickly corrected and stopping the action. As far as whether I think his aggression is fear or naturally dominant, I would have to say that I think it is more of a naturally dominant personality. However, this is just a guess. If he was subjected to a lot of aggressive loose animals while he was tied for those years, I would think their would be fear first, then then possible develop into a naturally aggressive behavior because he knows no other way and only thinks about letting the other dog know the "he is in charge and don't mess with him". A lot of guessing, but when we play with toys, etc., he does try to hold onto his toy without easily releasing it, so I assume that is a form of dominance. We also use the "Nothing is free" method a lot. If we give him a treat or play with a toy, he has to sit or down or something like that for the treat/toy. Thanks again for your input.
Marge
Congratulations on everything you have so far accomplished with this dog! You've done everything right and then some. Dog-on-dog aggression can be complicated. In my opinion, the question, "Why is he doing this" isn't helpful. You can't apply human psychology to dog behavior.
Can you determine if his aggression is due to fear, or from a naturally dominant personality? What was your trainer's opinion that?
While your Golden has come a long way, he may not yet entirely trust that you are his pack leader and can protect him. I had a dog just like that, and it took me years to figure out that her aggression was due to fear. She felt she had to be on duty against people and animals 24/7.
One important question I have is how do you handle the situation when you're walking and he goes after another dog? Obviously you're very good at anticipating when a conflict will arise, but avoiding it won't help him move forward. He has to reach the point that he can trust you enough to protect him in a situation where another dog is a threat. Right now, ANY dog is a threat to him.
I was able to get my dog to ignore other animals on our walks, but it took several weeks. Like you, I had to always be aware of our surroundings. As soon as I saw her intently looking at another dog (say across the street) I did a pop-release with the leash and kept encouraging her to move forward. If she was allowed to stop, stare and issue a challenge, that made for a possible fight. After a few weeks, she automatically looked to me to see how a threat was to be handled. That was a wonderful day the first time it happened!
A short, but loose leash to allow for correction is imperative with these kinds of dogs. Allow just enough room for a pop-release, and don't allow your dog to move ahead of you. That empowers him to think he is leading the walk.
Since you've done so well with him to this point, I think it would be worth your time and money to have a behaviorist work with your both on this issue. Sometimes just having another person (trainer or behaviorist) look at a problem will help you find a solution.