Should I be worried about my younger brother and forcibly have him committed?

We had a recent death in the family. My brother is now suddenly heavily into the Goth/Emo subcultures. He dresses the part listens to the music. I understand that this may be his own unique method of dealing with his pain. However I found his journal. Alot of his creative writings are about him wishing he could join our parents in death. I know I've committed a huge breach of trust by reading thoughts that were meant for his eyes only but I do not want to lose anyone else. I've also seen unexplainable injuries on his arms and legs. When I ask about them he tells me that I'm not his father & to mind my own business. I feel I should have him forcibly committed for his own safety. Or at the very least get him to talk to a Psychologist. I can legally do this because I'm his recognized Guardian. With our parents dead I've taken custody of him because he's 15 and I'm 35.