That wouldn't be fair to the cats. Their entire lives, they've had the ability to go where they please, when they please. To force them to become indoor-only cats is akin to putting someone in prison for a crime they didn't commit. These cats have done nothing wrong and will view it as nothing but punishment should your husband force them inside for good.

Live and let live... if you interact with the cats outside and they obviously know where their home is, there is absolutely no reason to change anything now...