Hi maybear, it must have been terribly disappointing for you to find he's been drinking again, especially as he does seem sincere in his wish to stop, but found it too hard. As an ex-A I know it IS hard until you get to that state of mind when something just clicks, and you're ready.
Your AH may have thought staying sober would be much easier than it turned out to be. Your position is strong; you won't have drinking in the house, so now it seems to be a matter of seeing how determined and capable he is of stopping.
At least he now has a realistic idea of how easy it is to slip up, and the relapse may have taught him how to avoid it again. Time will tell.
If he's at his father's drinking now however, it doesn't look good.