Tell his parents what he said to you and suggest to his parents that they tell his shrink what he said to you as well. Then get yourself away from him and quit thinking that you can possibly do anything to stop him if he has his mind set on this. You can't help him by doing anything except relating to his family and doctor what he has hinted at. Then its up to his therapist to have him committed if they think he's a real harm to himself.
This is NOT your responsibility and as much as you love him as a friend you are not his keeper.
He is already assisted by a psychiatrist, has been since three years,
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion