Worried about brother - help!!!

I need help regarding my younger brother "Oliver". He is 16 and I am worried he is dangerous to himself, his girlfriend, and his family. He recently has gotten his first girlfriend and since then he has been absolutely obsessed. They talk on the phone for at least 5 hours a day and see each other before, during and after school. I am not exaggerating...he talks non-stop. Within 2 days they were saying "I love you" and when I said he was going fast he would either hit me or call me dumb or jealous. Since he has been dating her he has been saying that his life is worthless and he will go nowhere. I am worried about what will happen if she breaks up with him. He is very strong and benches 210 easily. Even though I am older then him I am extremely fearful of him. I do believe if she broke up with him he might hurt or kill someone. Please help me in coming him down... -Matt