Many roles. First, he needs to be there for anything you ask for. Ice chips, massage my back, just hold my hand. You will find if you have someone to distract you through the contractions it will be much easier as well. When you are in the pushing stage, he can actually help hold your legs up for pushing, so you can concentrate your strength on the push. He is your manservant that day!
Just be there. I think just having your husband there is a start. It is a very personal decision, but i think you should limit the number of persons with you to your husband. It gives a level of intimacy that is special and moreover it gives your husband the leading role in support person.