All the things you just said are textbook examples of things a person does before committing suicide. On top of that, he *said* he's going to commit suicide. And even if you have doubt... which is better? You not calling, and him dying? Or you calling, and it turning out to be fake. I choose call; better safe than sorry. You will regret it if you don't.

I realize calling 911 is a scary thing to do, but the operators won't take offense if turns out to be false, no one will. He told you he was going to commit suicide, and you acted on that assumption as a friend. They wouldn't care if it turned out to be fake.

p.s.
If you're still unsure about calling 911 about it, you should talk to your parents about what is going on and bring up the possibility of calling 911.