Do I have any recourse

My husband and I have been legally separated since April 12 and we have joint custody of our 5 year old son. I found out on Saturday that his girlfriend has moved in with him. It is in our agreement that there will be no overnight visitors of the opposite sex unless they are on the mortgage or the lease. I have asked him to show me the lease with his girlfriends name on it and tell me what her last name is. He is refusing. He has told me that I will have to take him to court to gain access to this information. I would like to have the terms of the custody agreement changed. Do I have any recourse?

Your best option at this point is to file for breach. It may end up being fruitless, unfortunately, as he can just add her to his lease before the contempt hearing date.