I know how you feel! Me and my fiancee do everything together, we've never kept a secret. We always give our xmas presents like a week early because of we can't keep anything a secret!
The truth is, you and your fiancee know each other. If you show him a picture, he can imagine how you will fill it out. I know he hasn't seen you veil or make-up, but the dress it the most important. Think about how other grooms faces changed when they saw their bride for the first time! That look alone should be enough to stave off you urge to show him. And besides, everyone knows things are more interesting when they are kept a suprise. I think if you did show him, you would be disappointed.