I like the idea of starting with once-a-week weigh ins/prizes and then maybe working up to percentages lost once your husband warms up to the idea of the weight loss challenge and you both want to make it a bit more challenging. Or perhaps have "bigger prizes".
Prizes could be things like, cooking for the other (maybe a favorite treat that's been modified to a diet friendly version or a cheat meal), at home pampering/spa type things, maybe taking care of the kids that night so the other has a night completely to themselves for relaxing or can have a night out with friends. Maybe a bigger prize could be a date night for both of you to celebrate your losses, the "winner" chooses the restaurant or location.
To change things up a bit, you might think of the opposite and some weeks you could have things the loser gets stuck with, like getting stuck with certain chores for the night.