Someone really ought to figure out why it's so easy for some people to maintain a healthy weight, and so difficult for others

I've given a lot of thought and research to this question over the past couple years and I'm pretty confident that John Walker's explanation (as set forth in The Hacker's Diet) is the primary reason. Specifically, your body has a natural feedback system -- if you do not eat enough, you have the urge to eat more. If you eat too much, you have the urge to eat less. People who naturally gain weight have feedback systems which do not function properly in the modern environment. They don't get strong enough signals to stop eating so they have a tendency to over-eat every day and they slowly gain weight.

In your husband's case, when he pigs out, he has a natural urge to eat less which results in him maintaining balance. So he can eat whatever he feels like without gaining weight.