Deciding that returning to his teammates with an obvious chubby with how tight Jeff's clothes were on him would be something they'd never let him live down, Mike dropped the shorts and decided to do something about it. He fumbled with the lock on the bathroom door--he didn't want to be interrupted by anyone after all--and then leaned back against the door for support and took his right hand around his swollen cock while his left cupped his sack. His mind began imagining a naked girl inviting him so her bed, but then it wasn't him joining the girl, but another guy--one which Mike couldn't discern as anyone he know, and yet who seemed extremely familiar--and as the two tore their clothes off of each other, Mike continued to fap away, harder than before. Precum soon coated his right hand, making it easier to slip and slide up and down his rod. The guy and girl were in the deep throws of sex now, and here was Mike watching in his mind's eye. He grew excited as he rammed her, and as she gripped and groped his muscles. And soon as Mike had slide down to the floor did both he and the couple in his imagination erupt in orgasmic bliss. A large splotch of jizz landed from Mike's dick and onto the floor, with his cock still dribbling a bit more thereafter.
There he sat on the tile floor for a few moments, reveling in the ecstasy of the moment a bit longer before the dormant sensible side told him he should start to clean up now, before his chum hardened and proved difficult to clean up. And so, reluctantly Mike stood up, feeling a bit disoriented when he did, as though his sense of balance had been shifted slightly. He grabbed some toilet paper and started first with cleaning his own hands--that seemed oddly oversized compared to the arm they were attached to, but Mike dismissed that thought as just not being used to seeing his teenage body's proportions in a few decades. It was however when he cleaned his shrinking shaft that Mike gave a pause. His dick and balls felt somewhat... smaller in his hands than they'd felt a few minutes ago. His dick he could understand--he'd just rubbed one out after all--but his balls? That seemed strange. Once his body was cleaned up, Mike put Jeff's shorts back on. They now no longer felt so snug as they had a few minutes ago, but instead felt a bit loose on him now. Maybe his erection had stretched them out. It didn't matter, he had a cum puddle to clean up first.
After at least three wads of toilet paper--the puddle seemingly larger than Mike had remembered--the last of the seamen soaked wads were flushing down the toilet. All that remained was cleaning up the residue from the floor and his hands, which was a few squirts of a liquid cleaner and a washing of his hands and Mike looked up as he reached to dry his hands and nearly fell over in shock as he properly looked in the mirror.
Gone was the 6'2" muscular High School senior starter from the football team. In his place was what looked to be at most a sophmore, maybe even a freshman standing at 5'5", with a much leaner build, a messy mop of hair, and who looked far closer to the start of puberty than the finish line of it.
Unbeknownst to Mike, but by ejaculating out his adult jizz, he'd fapped away a good few years of his development. Jeff's clothes looked far more normal on Mike, even a little bit big on him. And it was with that realization that Mike realized he'd gone from being older brother to younger brother to his own son with one jerk off session. He had to warn the others!
Mike unlocked the door and hurried back to Jeff's room, only to be greeted by what looked to him to be intimidating giants who all looked jittery and whose erections were plainly obvious to everyone in the room. A distinct musky odor now permeated the room the moment Mike rushed in.