Many men have fantasies. Spanking, domination, cuckolding, being forced to serve her while she is on a queening chair, etc. However, reality is often different than fantasy. Spankings hurt. Domination may hurt physically, or in some cases mentally (pride, ego).
Cuckoldry may hurt is pride via the humiliation of another man taking his wife and of course oral servitude is rather gross be it vaginal secretions, another guys sperm or being ordered to kiss and lick his wife's ass. So they often then try to back out of it, barter, bribe and whatever. The fantasy is still there, but the reality is more intense than they imagined.
Just do him. Take charge. Insist, order, demand, assign punishments for failure to please you, in whatever manner, and follow through with those punishments. It may be orgasm denial, chastity, spankings, ass fucking him, denial of sex, whatever until he accepts his submissive stature and deals with it. He wants it, he's told you that.
Maybe only some parts of it, but now decide what parts will make YOU happy and satisfied and go with it. Good luck. :)