Prod - great advice. If you call 911 every time, you will either call his bluff and he will not try it again or it will get him the help that he needs at that moment in time.
My counsellor prepared me for this when I spoke of leaving. The biggest message he gave me was that if he did do it, then it would not be my fault. And that staying because of that fear would not be acting in my best interest in any way.
Btw, 999/1000 it really is just a last ditch attempt at control and one of the sickest methods of blackmail, imho. Especially when the best way to get a spouse to stay is to get treatment for their drinking issues, not kill themselves. Just another sign that the prospect of recovery is still some way off.