As mentioned before, subs love the loss of control that comes with being tightly bound and gagged. Just make sure they cannot get out.
Keep the knots out of reach and make sure they can't even squirm around. There are many resources on the web for different bondage positions.
For some added fun and humiliation, tie him up and leave him in a closet!
He will be left in the dark, horny, helpless and wondering what will happen next.