I think -- by and large -- teenage guys care more about image than anything else, because self-worth is tied in to pretty much everything in their life -- friends, love, power, etc. How you are seen determines pretty much everything.
The guys at the bottom are treated like dirt (usually badly bullied), and the guys at the top are looked up to. Nobody wants to be at the bottom. You're also more likely to avoid getting rejected by the girl you like if everyone looks up to you. How 'in demand' you are affects the importance people place on you, and so being some kind of social pariah will usually get you rejected, even if you may have stood a chance under another situation, simply because your LI doesn't want the social stigma attached to being an item with you. So. Anything to do with public humiliation, or being put down, or feelings of helplessness will make for great 'male' fears.
I'll talk a bit more about rejection. The shame of rejection is every man's fear. Not only does it hit your self-esteem because yet another person doesn't want you, it can also be humiliating when word gets out, because other people know it too. It gets worse if they find it funny, or think that you were an idiot for thinking you had a chance. Maybe it even makes them think less of you. Either way, it hits your self-esteem bad (and, at that stage, your self-worth -- because most people don't become certain that they are worth something regardless of what people think until much later).
Guys want to date their crushes, but it takes a lot of courage to ask them out, and there's usually this long dance where a guy tries to find out how a girl feels about him so he can know if he'll get shot down or not. If he's going to get rejected he doesn't want to get rejected and lose face doing it. Which is why a lot of guys do the assholish things they do (i.e. taking the piss out of a girl who rejected him afterwards, in order to shrug it off in a "she's not that big a deal anyway, I don't care" way), but these are usually really immature. It doesn't help that most girls are so tight-lipped, even when they're interested. Haha. I guess they're usually trying to save their own pride.
So anyway, to sum up: shame (especially public) and helplessness/powerlessness (especially public). It's stuff everyone can relate to, really. So...
*Being the one to ruin a crucial game in whatever sport. It's the school's one chance to pass through into the finals, and he's just joined the team, and he blows it. He's the reason why they lost, and everyone knows it. Everyone is angry and feels that he should just quit.
*Asing a girl out in public, after pouring lots of money and effort into it. Maybe in the middle of a game, when everyone is sitting on the sides. And the girl puts on a disgusted face and says no. Then maybe her dad hits him and tells him to stay away from his daughter. Afterward, everyone calls him a creep.
*Someone punches him in the balls (nooooooo)
EDIT: This is probably important for anyone else writing FOR guys: Most guys I know want to feel important. Action -- where the main character shows up in a really cool way at the last moment -- is cool because things turn around, but also cool because the MC is important. They're crucial to the way things pan out. It's that self-worth thing again. And power. Fantasy stories where a single person changes the outcome of an entire war... any form of badass... etc. All these action tropes are usually related to how other people view the character. They're capable and admired and useful. They can do things others can't do. I think that's what a lot of people get wrong when they write guy characters. They don't put enough emphasis on self-worth and image, and then they come off as not-"guyish" enough.
It's also the reason why introspection and so on don't get as much room in the mythical "guy" book, because it's not so much about how the character changes as how the world around them chances (especially when it comes to how the world views them).
