Okay, here's my thoughts... take them as you will.
My younger brother was the exact same way when he was 12-14. The exact same way, down to the letter. In fact, he even pulled a knife on me once, too - and I responded by pulling one of my own - there's no telling where it would've gone if my mom hadn't walked in on us. My brother threw things, too - I have a long scar on my upper lip from when he threw a bucket at me and it split the lip up into my face, one on the lower lip from where he threw a vacuum cleaner tube, and I almost got killed once when he threw a heavy wooden chessboard at me (it was thrown with enough force to punch clear through two doors, so I can imagine what it would've done to my head). Violence is definitely not the answer, I can tell you that much. No matter what you do, it will escalate. If you pull one punch, he'll pull two. If he pulls a knife and you pull a second one to keep him back, he could very well throw it at you. It's important, of course, not to look weak or in any way make him think he can get away with whatever he wants (don't let him think he's "top dog", basically, or you'll never hear the end of it), but I would not use any display of force if I were you.
What I would personally do if I were in your situation and he pulled a stunt like trying to annoy me: I would ignore him. It'll be hard at first, especially if you're used to responding to it. The knife thing is him simply trying not to feel defenseless. Unless he actually lunged at you, I seriously doubt it was his intention to cut you with it - rather, he was trying to keep you away. In my case, my whacking my brother was entirely due to him picking on my youngest brother, making him fetch things and clean things and being all-around lazy while he expected the youngest to basically be his slave/footrest (not literally, but it should give you an idea of how he treated him). Calling on authority wasn't an option for me, nor was getting him to an expert.
One phrase that did work really well, though, as I recall, was, "Seriously? I thought you were more mature than that." He's growing up. He wants to feel like he isn't powerless. He wants to feel in control, and he wants to feel like he's more mature than he'd been before he started all this. If he can get a reaction out of you, who he sees as the higher authority, to his subconscious mind, it means he's that much more "powerful". (I'm not going to college for this so I may be using incorrect terminology throughout, by the way.) When I insulted his maturity AS AN ASIDE, perhaps rolling my eyes and turning back to my work like what he was doing didn't really matter, he lost that sense of "power", felt a little ashamed of his actions, and felt somewhat motivated to clean up his act.
Another thing that's important is not treating him like he's "just your little brother". If you treat him like you're head and shoulders above him, and he's just an ant compared to you, he won't like it. He'll fight back. He'll try to defend his "manliness", and you've seen that manifest itself in how he pulled that knife on you to defend himself. Treat him a little closer to an equal, where you can. Don't pick on him. Seriously, my brother and I used to be mortal enemies, but now he and I are incredibly close and practically inseparable. His personality changed itself as well, and he's a much better person now, even though he's still rather cynical.
This isn't going to happen overnight. It'll be a rough road ahead, and there will be more confrontations, but if you start treating him better, things should clear up eventually. Remember, you're personally helping shape who he becomes when he's older.
(Professional help is a good thing, though. Never dis the pros. If they make a living off of it, there's generally a reason.)
edit: Tiruin had some really good points there, too.
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Topic: Little Brother problems (Read 4663 times)