Just tell him you need him to pick you up some things from the store and when he asks what, just have a list prepared. He will get the hint. i had a hard time telling my mom, so i bet it will be even harder for you to tell your dad. Your dad won't know enough about it to help you, so it probably wouldn't do any good to ask him about it. But I am almost positive he would get the hint when he looked at the list. Just give him the list and get yourself out of there before he reads it, lol.
Or even better, email him a link to this page and tell him it is a question you asked.