
Carringtons post is spot on..
He doesn't need to run and play with strange dogs, and thats not just an opinion.. its been studied and dogs do NOT need the company or interaction with other dogs - its us humans that like to see dogs playing with other dogs, and expect dogs to be friendly and playful with other dogs they have never met before.
What your dog NEEDS is to respond to you and desire to be with you and work with you above all else.
After that a dog needs to be comfortable and relaxed in the presence of other dogs.
After that, well anything else is 'nice' but not necessary - ideally your dog will be fine to be introduced to any dog you wish to introduce him to, and ok to walk alongside a strange dog without feeling the need to interact or behave fearfully or in a frustrated manner.
The problem with the advice 'socialise him with lots and lots and lots of other dogs' is that you end up as I think you currently are doing, seeing every walk as an opportunity to meet, greet (and from his point of view, physically and mentally test every dog) other dogs - and when that becomes the goal, the imperative to work with you and bond strongly with you is lost, it gets put on the back burner.
I realise a lot of the above might sound draconian and 'mean' but the reality is really no different to your own - YOU wouldnt expect to rush over to a total stranger and start pushing them around or getting physical with them in any sense. You wouldn't like it if strangers always did that to you either. But you can form social circles with the people you meet at work, in various hobbies etc...
So can your dog, so then your dog is fine with the dogs at dog club, hes ok with relatives dogs and maybe the neighbours dogs, or a group of dogs you regularly meet and walk wtih at the park - but he DOESNT feel the need to dash off to meet every dog that hoves into view, because you make it much more rewarding to stay with you, and you set up a cast iron habit of waiting to be introduced before interacting with another dog.
And if everyone did this, we would not have any dog to dog aggression problems!