not much. just be there to support him in what ever choices are made for him. as every woman knows that it is her wedding and all he needs to do is show up.
if the bride to be wants you to help do what ever she wants. just do not do anything that can upset the bride as she will hold it against you for the rest of your life. believe me when i say this, it happens all the time.
please consult a wedding consultant or a book on weddings to see what the grooms family is responsible for. I think there is a for dummies book on wedding out there.
not sure if this will help Good luck