Love the idea of the unawareness. There's a story on CHYOA called My Oblivious Mother (and a few others there, or on Literotica from CraptainPlanet and Captain_Ron) that banks on it, you might look to for inspiration for scenes the son might notice but not appreciate for what they are, or some mind trick the bully can pull off on them, making them oblivious regarding shame or embarrassment when he's involved, or the always-favorite, power that makes people really gullible and naive regarding what you're saying, "Oh man I got bit by a snake on my dick, can you suck it out? If you don't get the venom out, I'll die!" Though I appreciate edging would last much longer. Maybe the MILFs are in a hot tub and the son sees one of them sitting higher than the others, figures there's some sort of raised seat. Next time he looks it's another mom sitting on it. And then another. And then all three moms are just lounging in there while bully sits back where the 'rise' was, seemingly no higher than the others.