have y'all talked about counselling, or a weekend retreat, stuff geared toward helping your marriage by helping your focus on it without interruptions? Sounds like he's waffling because he doesn't know what he wants, and instead of stying put in one place to figure it out, he thinks wandering between two relationships will give him that answer.
just got this from a priest friend, about a retreat for couples in hurting marriages,
www.retrouvaille.org
anything you can do to help him focus on the problem will help ...
good luck ...