OP, could you tell us a little more detail about the situations in which he barks at men? Is it all men, or only certain ones? Where does it happen--in your home, at the dog park, etc.? How do they introduce themselves to him? How close do they have to be (or do they have to be approaching/paying attention to you) before he reacts? This might help get you some better answers.
In general, when I have seen dogs (especially young puppies) be "aggressive" towards men, it's fear-based reactivity. Men can be more threatening to a dog because they often are taller, have deeper voices, and are generally socialized to have a more assertive/aggressive body language than most women (obviously all huge generalizations). This is something the dog may grow out of, but it's important to nip it in the bud and not let the dog continue to practice the behavior as that will only make it worse.
Without knowing more about what's going on I don't have any real training advice, but I do recommend that you consult a professional trainer. Reactivity is not something to take lightly and if not addressed, it can lead to serious problems like biting. Even if it isn't fear-based, the behavior is apparently out-of-control (even a protective dog--which most puppies aren't, they're just kids!--shouldn't be behaving that way in every day situations) and needs to be handled. This is not a breed issue or a male dog issue, it is an inappropriate behavior.
As you look for a professional trainer to help you, try to keep him out of situations where he will be forced to interact with unfamiliar men. If he is reactive, forcing him to socialize with them is probably putting him over threshold and just reinforcing in his mind that they are scary. There is a way to socialize dogs in these situations (especially young puppies), but it needs to be handled carefully so as not to make the behavior worse.
Originally Posted by Lauri & The Gang
Instead, I would teach him that when you approach him while he is eating GOOD things happen.
I have my steps for dealing with food guarding outline on my website:
That is a really good method for dealing with food aggression. I've done similar methods and had great success. Thank you for writing it up so clearly!
OP, I'd do Lauri's method. Definitely stop trying to make him step away from his food bowl. When you do that (at this point), you are teaching him that you can't be trusted around his food and likely actually reinforcing his resource guarding.
I also agree that arbitrarily taking things from dogs/making them leave their meal/sticking your hand in their bowl isn't really okay, unless you're teaching a "leave it" or "out" command (in which case, it's not really arbitrary). It does annoy even most tolerant dogs, and for good reason--how would you feel if someone kept reaching over and stirring your dinner for no reason? It's just rude.
Instead, use Lauri's method to teach the dog that you messing with his food is a good thing (because he gets more food!) and then don't abuse the privilege--just keep adding more food by hand occasionally to reinforce the training. Never make him leave his meal unless you've got an actual reason (like you set the bowl down and as you do so you see a contaminant in it or something, or he gets hold of something he shouldn't have).