There would still need to be an awful lot more information understood about Duke, his behavior, and the circumstances when the behavior arises in order for someone to help.
Based on the limited info however, it does not sound like a socializing issue at this point, but rather one that now requires "counter-conditioning". Socializing by definition is introducing a dog to various stimulus and things to classically condition them. Once negative behavior has taken hold, then it is a question of counter conditioning that behavior.
It may be that he did not get enough socialization, and efforts there may help. But a dog should never be offered the chance to "practice" their bad behavior. For example, if he is lunging and growling at dogs, he shouldn't be taken out around other dogs and put within the threshold to do this - even in a muzzle....it is a reinforcing behavior so every time he does it and processes it in his brain, it gets further solidified.
Your best route to turn this around and retrain him is to work with a certified CPDT trainer who can give you a good counter conditioning program. You can look for a trainer in your area by searching at
www.apdt.com and look for those with a CPDT certification.
Short of that, you could also read some helpful books and pamphlets if you want to try to go it alone. The best guidance on the subject is "Feisty Fido" by Patricia McConnell. If the root of the behavior is fear however, then you should also consider "The Cautious Canine". Hence why a trainer is critical to getting the diagnosis right (is he fear aggressive, aggressive, or just reactive for example - different approaches potentially).
