Work the finger in and out a little, then slide another one in and continue teasing his ass. Tell him that you wonder how many fingers you could fit, and slide another one in. With each additional finger, let him know how impressed you are, and how hot this is making you. Moan, writhe, bite your lip.
When you work the last finger in, Tell him how much you enjoy playing with his ass. Ask him if you are making it harder for him not to cum. At this point, he will most definitely say yes (did he say Mistress?). Make a pouty face, and say, “But I don't want you to cum yet! Here, let me help.” Grab the plug, lube it up in front of him, slowly move it between his legs, then pop it into his ass. Tell him to thank you.