Could thank him for coming to you with his thoughts and feelings. That's what spouses do, right? He's not cheating or anything. He's disclosing. Maybe he's scared to say it to you -- just like you are scared to hear it.
Could be honest back -- that this scares and terrifies you. Could tell him what you do NOT want. You don't want to make any changes to marriage agreements at this time.
Could state what you ARE willing to do at this time. Maybe read articles and books. Talk about his feelings. Get to know this side of him.
Could choose to Open to more of HIM even if you don't Open the marriage at all.
It's your marriage too -- you don't have to do things you do not want to do / are not willing to do.