Fuck how you tell him
From now on you have the key to his orgasm. Tell him that every time he succeed a task, he deserves the right to orgasm. Every time he fails, he deserves punishment.
You will do a "cum in 20 seconds" week. You will be fully dressed but with your latex outfit under your clothes. Normally he wont succeed (if he does try again with a "cum in 10 seconds" week). So tell him you're gonna help him. Buy some anal lube and tell him to use one finger to jerk his prostate. Normally it will help a bit but he wont make it (if not "cum in 10 seconds" week). so tell him to use two fingers. Then buy him a prostate massage tool to help him.
So this is the forth week of your fifth month as a mistress and I have a gift for you. Tell your boy it is TEST week. Deny him the ability to orgasm during five days then with the strapon you have bought on the internet fuck him. Fuck him deep. After a few moments, grab your watch and grab his cock and jerk it the hardest and fastest possible. Normally he will cum very quickly.
"Oh I didn't know you are so much into anal."