The fantasies going through my mind lately have mostly been harem or free use stories. All of them involve the boy with a mother and her daughters. Either he's staying with his aunt and cousins for the weekend, or there's a neighbor's house he frequently visits to get sex. Either way, clothes come off at the door. Everybody is naked and he's fucking the shit out of all of them, over and over, never losing his erection, splattering the house with used condoms and semen.
The one element that turns me on in these scenarios is the mom character is completely moral free. I pictured the aunt coming home to find him fucking his cousins, and the reason she was gone was to buy more condoms. Used condoms are already all over the fucking floor and furniture, by the way. Of course, when the condoms run out again, he gets to rawdog them. And the mom character gives no fucks. That part of it gets me hard more than anything.