What type of obedience classes did you attend and what kind of techniques were used? If your dog was "corrected" for growling at a man it can actually increase their fear of men and thereby their aggression towards them since in their mind something unpleasant happens when men come around -"I get "corrected" when men come around and it hurts/frightens me"; they are bad news! And if they do get to the point where they are no longer growling it doesn't mean they are no longer feeling aggressive it just means that giving warnings have been sufficiently punished enough to make them stop giving you a warning-in the words of Dr. Ian Dunbar-you have removed your ticker from your timebomb.
You do need someone experienced with counter conditioning and desensitization methods and using non punitive training methods.
www.iaabc.org was mentioned.
www.trulydogfriendly.com is a good sight for positive trainers. I have put a site about aggression below for you as well. Using a Gentle Leader can give you more control over his head, but find someone well versed on how to properly introduce and use them. We need to change his perspective that the presence of men mean great things happen. You might consider getting the book The Cautious Canine by Patricia McConnell it covers counter conditioning and desensitization. Dogwise.com has good book/dvds on aggression the website is below.
I wish you well because such a dog does need a lot of management and dedication to help them since such behavior can never be "cured" since that implies it will never happen again. But you can hopefully get them to a point of manageability and a certain amount of confidence so they do not feel as threatened since most aggression is caused by fear.
I would also have him checked by a vet to rule out potential health problems that can contribute to aggression such as Hypothyroidism, early onset arthritis (if he a younger dog), arthritis (if he an older dog), Diabetes, and seizures for example can and have contributed to aggression.
And unfortunately, some dogs can have neurological damage. Distemper, for example, can cause a high fever and brain damage, or if a victim of physical abuse he could have lesions caused by blows to the head or severe psychological damage caused by the abuse.
I have worked with many aggressive dogs over the years, and there have been 8 I choose not to work with since I felt that I would not be able to help and referred to the Behavior Clinics at U. C. Davis or Berkley. So 8 in 10 years is encouraging since such dogs are the exception rather than the rule. The others cases have been successful and some of them had bitten previously, so there may be hope.