Good girl gone bad

I read your post and find it interesting, because I would love such a scenario myself.
As for me I would suggest you just take it slow. He opened up to you about his fantasys, but that doesn't mean it HAS TO BECOME reality in an instant.

Let him out and end your episode of denial and chastity if he gets whiny and wants out. You'll soon see, that he wants you to take control again... and start from there... Its more of a progress, than a decision for someone new to this. Or with a new partner.
After all it comes down to trust and respect.

Sure, everyone might say "he is not serious about it if he gets all whiney etc.", but I guess he is just coping with the transition.

Just take it slow... life can be very long if you enjoy it with someone together - so there is now rush!