Yep I've used calming supplements. They doesn't work and show up on drug tests, lovely stuff.
The best way I see to fix the problem is don't turn him out near, or with mares. That way he can't mount them, and his aggressions will probably cease because without estrogen around, there's usually no reason for boys to fight :P (The same thing applies to male humans)
Thanks everyone. He hasn't been turned out with any other horses except my mare (who he mounts but doesn't herd or fight with) for quite a while. If there are horses turned out in the pasture while he's in his stall, he'll squeal and strike out and buck over and over. If he sees other horses from his pasture and they don't come over to say hi to him, he'll go crazy trying to get their attention and end up drenched in sweat. He got his leg stuck in the fence during this a couple weeks ago, and that's why I decided to try something. It doesn't really matter to me if it shows up on drug tests because I don't show him.