I also want to mention that him withdrawing, on his own, over and over is playing with fire, although I'm sure many alcoholics do this. At some point he may have a seizure or worse. Just be prepared (sorry, not trying to scare you), if he seems to be flagging during one of these withdrawal sessions, don't hesitate to call an ambulance, better to be safe.
Just because he has never had a seizure (that you know of) doesn't mean he won't.