Synopsis (or, at least, the relevant part):
“Not foalcon. Not incest. Aged-up.
Sand, surf, and sex. A teenage Button Mash goes down to the beach, looking for something to occupy his time. He meets three mares, and just as his luck would have it, they’re all in estrus and can hardly keep their hooves off of him, not that he minds. Sometimes, the best way to beat the heat is to embrace it…”
Yeah, you can tell what happens next.
Source link leads to the fanfic.