Some dogs just don't like other dogs. And considering your rescues history, why force him to socialize with the very thing he fears and hates the most?
As long as he's good with people, I would focus more on managing his fear-aggression towards other dogs.
You can teach him the 'Watch' or 'Focus' command, which you can use on walks to get him to focus on you and ignore any other dogs he sees.
Does he go nuts (barking, growling, lunging, etc) when he sees another dog? If he does, you can use the LAT (Look At That) method.
Look at That! A Counterintuitive Approach to Dealing with Reactive Dogs Dog Training for Dog Lovers Blog
This basically teaches him that he doesn't need to erupt when he sees another dog and he can ignore the other dog. You can start teaching Dock the basic method with his toys or something. When you see him look at the toy, say Yes! and treat him. Every time he glances at the toy, say Yes! and treat. Eventually he'll start offering the behavior, the looking at the toy, on his own. That's when you'll want to "name" the behavior. As he looks at the toy, say Look!, and then treat him.
Once he understands that you saying Look! means you want him to look at something, then you can move on to actual dogs.
You start off with Dock as far away from the other dog as possible before he starts reacting. You want him to be "sub-threshold" or under his reactive line so that you're not rewarding him for reacting. When you see him look at the other dog (but NOT react), you say Look! and treat him.
You slowly and gradually progress closer and closer to the other dog, as LONG as Dock is not reacting. If he does react, you've gone too far and need to do a quick u-turn away from the other dog and keep walking until he calms down again. Then you start the process over again.
At this point, Dock doesn't like dogs and probably never will. He's had too much of a negative history with them. Socializing him with other dogs can be dangerous for Dock and the other dog unless handled and observed by a trainer who knows what they're doing. Focus on management and love your dog for who he is, not what you think he should be.