J.,
I used a gate on the bedroom door of all of my three kids now ages 14, 9 and 5. I told them, "It's not safe for you to be up wondering around the house at night when Mommy and Daddy are sleeping. This gate is here to keep you safe in your room. If you really need me, call me, and I'll come." Then, just like trying to get a baby to sleep through the night, when he calls, don't go in immediately, take your time.
I think the other mom who responded had a good point, don't interact too much with him at night. Mimimal words, no play and if you rock him, keep it short. He's wanting you to interact with him in the middle of the night so don't feed that hunger. Also, I always gave my kids a shirt of mine to sleep with so they could smell me while we were apart at night. My 5 year old still sleeps with my shirt and she snuggles with it.
Another local resource is child psychologist and parent coach, Tina Feigal. She's awesome, I know her personaly and highly respect her methods. Her web address is www.NuturedHeart.com She has a lot of articles and info on her site.
Best of luck, hang in there, we all go through this and I know you can tackle it. One day you'll say to your husband, "Remember when he didn't sleep through the night? Aren't you glad those days are over!"
T.