Boundaries are about what you will and will not tolerate in your life. They have nothing to do with changing his behavior. That's what makes them to tough to enforce. You cannot "make" him do or stop doing something. You can only decide what you will do if and when something you cannot tolerate happens. So, if he drinks when he is alone with the kids, what will you do? That's the hard part. Whatever it is, you must be willing and able to do it, otherwise your boundary means nothing.
L