my boyfriend had suicidal thoughts 😰😥

Today my boyfriend really scared me. He has some mild depression due to some family issues. He mentioned to me imagining himself hanging himself in his closet and his sister finding him and leaving a note behind to his parents. He was descriptive as if it was planned out, but he told me not to worry because it was just a thought and that he wouldn't do that at all. I don't know what to do. Is this a sign of something more serious? Should I tell someone? I'm really afraid of what might happen 😰😰😥😰what should I do? 😔😔