Need advice from the menfolk :-) Or anyone, really...why is he doing this?

He's just trying to make sure he has a back up girl in case the one at work doesn't pan out. Open your eyes ; he is playing you. Best to forget this one and thank your lucky stars that his vacillating occurred prior to a marriage and before any children entered the picture. He is not relationship material. Think about it. He is currently seeing someone else and calling you to make sure you aren't giving anyone else play? Foolish of you to even consider him as a friend. He's not.