I see where you are going with boundaries, or the direction you want to go...however there is a difference between Rules, evidentiary proof and Boundaries.
boundaries are for us. that you and the children are now living in another state indicates you could no longer tolerate HIS behavior and then YOU took action. ie LEFT. you were not willing to tolerate living with active addiction and so you removed yourself from it. well done!
so now you are exploring possibly returning to where he is and attempting to resume the family relationship. and you want to make sure he is no longer drinking and making some type of self improvement first. the problem with distance is you can't SEE with your own eyes what is going on, or what appears to be going on. you will only have his WORD.
even if he started a program tomorrow, or last month, any recovery takes TiME. time to take hold and time to be lived out. when I say time I mean give it a year, at least. a year of repeated consistent behavior with no relapses. while a year is no guarantee, it's a very good start.
if he is motivated to change he will. if he wants to prove that he is worthy of being a reliable father and a husband again, he will. if his desire is to get sober NO MATTER WHAT, he will. and nothing will stop him. if he truly is enrolled in the program, he should have no problem producing whatever evidence to prove that. he would be an open book.
enrolling in a program is only a start. a month of sobriety is only a start. don't be rushed into changing your life and structure as it is today. you and the children deserve stability and security.