Your dog will probably always be somewhat dog aggressive. It may be a result of something he was taught by his previous owners. You're doing the right thing, though. He needs to be socialized, just as you are doing.
Try to distract him with something, a toy or even a small treat, when you are approaching another dog. If he responds to you and pays little or no attention to the other dog, praise him, pet him lavishly and, if you're using treats, give him one.
Remember, it's not just a matter of correcting the behaviour you want him to stop. Rewarding him for doing what you want him to do will give you a much better chance of restructuring his behaviour. Our dogs really want to please us (most of the time, anyway), so positive reinforcement is your most potent training tool.
Just don't forget, though, no matter how well things go, you won't ever want to leave him in a situation where he might be tempted to revert to his old, dog aggressive behaviour. He is a Rottweiler, and no matter how well mannered he becomes, if anything ever happened, he'd get the blame, even if he was completely innocent. I learned about that having German Shepherds. One of mine used to get blamed for things that happened in the middle of the night - when he was sleeping peacefully on the couch - in my living room!