First off, welcome to the forum!!
How long have you had the two together? If they are still in the "getting used to each other" stage, it may just go away..If not, you can just "stand up" for the mare and be aggressive towards him. Show him that YOU are the boss of the show and he is not. If you and your husband are both new to owning horses I would get a "horsey" friend or a trainer to come over and show you how to solve the problem. It isn't really something we can tell you how to fix without being there and seeing the problem first hand...I wouldn't want you to end up getting hurt if he turns aggressive towards you instead of the mare.
I am Sparkly Meanie Doodie Head and I approve this message!