In all states in this country a minor (person under 18 years of age) is deemed not legally capable of entering into a binding contract. Meaning, your soon to be husband can enter into a written lease for real property but he can always rescind it since he is under 18 years of age.The only exception to the above statement is if your soon to be husband has been declared by a superior court in this country to be a legally emancipated minor.There is nothing preventing your soon to be husband from signing the lease for the rental that you want to enter into. His mother signed presumably as a co-signer for him which makes her responsible for the lease's terms regardless of his age.Good luck.