Relax. It sounds as if you have a creative child. Children love make-believe play. Whether they're pretending to be the same gender, a different gender, a different species ("I'm a dog!"), or even a non-living object ("I'm an airplane!"), it's their imagination that's important.
Putting on his big sister's dress is in no way a predictor of his future sexual orientation or anything else, for that matter. You may want to remind your husband that such macho types as John Wayne and Arnold Schwarzenegger became popular heroes and extremely rich by putting on other people's clothes and doing make-believe for a living. And some of those clothes were pretty strange!
So instead of putting a stop to his pretend dress-up, I'd encourage it. Put together a box containing some extra clothing and props. Show him how a piece of plain cloth can turn into a cape that makes him a superhero. Join in on the fun. And try to get your husband to join in, too.