Start the day by giving him some instructions - email him the following letter with the subject line "private" to make sure he doesn't open it in front of friends, family or colleagues. Send him a text to let him know to check his email.
"Hi ______,
I hope you're having a good day.
I read the story you sent me from that sex site and am willing to give it a try. But ONLY if you follow EACH of the instructions I list below.
1. When you get home I want you to shower and shave your cock and balls - make sure you're extra clean!
2. Immediately go to the bedroom and insert a buttplug I left for you in the _____.
3. Get fully dressed and wait for me to get home.
4. DO NOT touch your cock!
If you do not follow each of these instructions I'll assume you're not interested and we'll forget I ever offered. But I really don't think it's much to ask considering this is something you've wanted for such a long time...
I'll be home at ___am/pm"