I used to train dogs professional, and have fostered many dogs.
I havent read all the replies yet (sorry!) but gonna throw in my 2 cents.
Has the dog actaully put his teeth on a human (made contact)? if so, did he draw blood?
Sounds silly, but it is important.
If he is just skittish around men, and acting aggressive that is easier to fix than a dog that has bitten and drawn blood. There are things you can do to desensetize a dog to people/situations that make them anxious. I would also contact a professional...but until you can get in to see them here are some things you can do to hlep:
1. make sure no male is intruding in on the dogs space. We want the dog to feel comfortable, not threatened by men, so it doesnt get worse. So most man need to ignore the dog as much as possible, and act non-threatenning
2. Have a man, sit on the floor, NOT facing the dog (sideways), and throw treats on the floor toward dog. do NOT make eye contact and do NOT smile. These are seen as aggressive moves by dogs.
3. never, never, never bend at the waist OVER the dog...that is threatening. alwasy sit or squat, and ALWAYS let the dog come up to you.
That will get you started. I helped work with my fair share of "scared' dogs. All of them are now happy and can go to parks ect, and you would never know there was a problem. It takes work and dedication, but it can help.
http://www.diamondsintheruff.com/ This wesite has a TON of free articles in the Behavior FAQ section. And there are a lot of good ones about "Dog Speak". Take a look a "space invaders" espically...and the "dog body language" so you can get better at picking out when the pooch is stressed, and remove him from the situation before he growls and nips.
This is a GREAT GREAT book about working with a fear aggressive dog: Emma Parsons "Click to Calm"
http://www.dogwise.com/itemdetails.cfm?ID=DTB825 and Patricia McConnell " Cautious Canine"
http://www.dogwise.com/itemdetails.cfm?ID=DTB586
They are both wonderfully helpful books!!!
hope this helps, and good luck!