After thinking a moment, the obvious choice came up again. His penis. He would grow his penis.
He dropped his pants and pulled down his underwear. There it was, looking somewhat small and mostly hairless. Two inches. It was the reason he hated taking showers in public. Most other boys his age were more developed than him. They had lots of hair. Jim just felt embarrassed by how immature he looked in comparison.
He looked at himself in the mirror, "Are you really ready to do this?" he asked himself. He hesitated a moment. "Yeah. Yeah, let's do it."
Jim took a breath to steady his nerves. "Control the size of my genitals" he said.
Putting the balloon to his lips, Jim slowly blew one full breath into the balloon, carefully watching the mirror. His small penis thickened and grew a full two inches longer as his balls swelled and drooped. It looked a little better than the other boys at school. Jim pulled up his underwear. Even though it was small, there was a definite bulge in his underwear. He smiled.
Slowly but less nervously, Jim blew another full breath into the balloon. The bulge rapidly filled out more as his cock lengthened inside his underwear, growing to six soft inches. Jim's balls swelled up more, becoming the size of large eggs. Jim turned to the side, admiring how his bulge stuck out from his body.
Forgetting his nervousness, Jim blew a full breath into the balloon. Jim's underwear quickly filled to the maximum and stretched as it struggled to contain a manhood far in excess of what it was designed. Parts of Jim's eight inch softie were now visibly outlined through the fabric. Jim grinned wide and danced happily.
Jim was suddenly curious. Just how far could he go? Apprehensively he raised the balloon to his lips, then hesitated. Did he really want to make it bigger? Yes he thought.
Taking the biggest breath he could muster, Jim blew as hard as he could into the balloon. Immediately, the underwear stretched more and more until...
RIIIIIPPPP. The underwear fell off, and out fell Jim's 11 inch cock. As thick as his wrist, it hung lazily down, besides balls the size of softballs. Jim almost drooled, they were so much bigger than any he had ever seen. Snapping out of it, he barely managed to whisper "release" to the balloon before waddling over to his drawer to find more underwear. Unfortunately nothing he saw could fit well enough. Jim sat down and wondered what to do.