Dustin was hesitant at first. He was hardly a champion cocksucker, and the fact that it was his foster brother Ty didn't help matters. He closed his eyes, trying not to look, trying to block everything out as he worked Ty's big dick over.
Ty moaned softly. It was hardly the best blowjob, but it still felt good. His oversized dick was a lot for Dustin to handle; Ty grunted occasionally, as Dustin's teeth brushed against the sensitive skin of his shaft. "That's it," Ty purred, "Get me going. Remember, more cum means more muscle." Ty set his power to reward Dustin's efforts. The more effort Dustin put into this, the more cum Ty would produce, and the more muscle Dustin would get. Given Dustin's less-than-stellar performance, Ty figured Dustin might gain twenty pounds.
Dustin grunted in frustration, still weirded out by all of this. But he thought about what Ty said. He had already experienced Ty's power first-hand. And he knew for a fact that Ty hadn't had such a huge dick before today. So if it were all true...
Dustin focused. He saw Ty's dick as a puzzle to be solved. A challenge to be overcome. He thought of his own dick, and the handful of blowjobs he'd received from girls at school, and yes, a few guys as well. He thought about the best ones, from a cute little gay guy called Matt. Remembering Matt's techniques, Dustin ran his hands up Ty's legs, caressing them, his thumbs running along the sensitive inner thigh area. Ty moaned. Dustin's hands continued upward. He found Ty's balls, and slowly worked them over, gently massaging them. Ty gasped and groaned. Dustin could feel the dick in his mouth quiver, could feel Ty's body tremble. Dustin reached up, pressing one hand against Ty's lower abs, using his other hand to gently squeeze Ty's balls. Ty gasped loudly, his body trembling. Dustin went for the kill, deepthroating the big rod in his mouth, bobbing back and forth with expert skill, as Ty's cock shook violently. A huge surge of cum erupted from Ty's tool, and he cried out, his eyes rolling up in his head as his whole body tensed up, every muscle focusing on the moment of orgasm. Dustin kept his mouth firmly planted on Ty's cock, gulping down the big load that shot from it, gently sucking on it, eager to get every last drop, making Ty moan and whimper softly. When he was certain that every drop of cum had been wrung from Ty, Dustin slowly pulled off. He sat back on his feet, then slowly stood up. He could feel something inside him...
Dustin staggered backwards, bumping against the wall, a soft moan escaping his lips. He felt so good... His muscles trembled, then began to slowly swell, growing under the skin, pushing up and out, getting bigger and harder by the minute. Dustin moaned with pleasure, his ordinary dick rock hard as he felt his body grow. He reached up to grab his own chest, panting as he felt his pecs expand, slowly growing into powerful slabs of beef. All over his body was changing, muscles growing, until he looked like a prizewinning junior bodybuilder--one of the larger ones, at that. The growth tapered off, and Dustin staggered away from the wall. "Fuck! What a rush!" he exclaimed, a huge grin on his face, eyes bright.
Ty stepped back. He had set his wish to make his cum production and Dustin's growth dependent on Dustin's effort--he just hadn't expected Dustin to put so much effort into it! He watched Dustin explore his new body, flexing this muscle and that, feeling himself up, reveling in his new strength. Dustin didn't even seem to notice Ty any more, he was so absorbed in his moment of self-worship. Ty took the opportunity to slip on some clothes. They fit, but they were snug in all the right places. Ty quietly slipped out of his room, leaving Dustin behind as he flexed.
It took a few minutes for Dustin to realize that he was alone. "Ty?" he asked the empty room, looking around. Not seeing his foster brother, he headed out into the hallway, still naked, and quickly walked to his own room. He locked himself inside, and headed straight for the full-length mirror on his closet door. "Damn," he purred, flexing slowly, watching his reflection repeat his moves. His dick was rock hard still, but he wasn't interested in taking care of it. Right now, he just wanted to admire his new physique. He found a measuring tape, and wrapped it around one arm. "Nineteen inches," he read off, under his breath. "Fuck. Gotta get to twenty!" He grinned at the thought of it. He stepped onto the scale he kept nearby. He had been tracking his weight to see how much muscle he was building. He laughed out loud as the needle swung up, passing his old 170, not stopping until it reached 230. Dustin looked in the mirror again, tensing his abs, grinning broadly.
Dustin wondered how big a second round with Ty would make him... His mind immediately flashed to the memory of Ty's big dick in his mouth. He didn't remember his initial reluctance, though. Only the pleasure of having a big, throbbing piece of meat in his mouth. And then the cum. And the growth. Dustin shuddered with pleasure as he remembered all of that. He wanted more.
He looked in the mirror again. "Fuck, gotta get to the gym. Put this bod through its paces!" he thought, as he and his reflection grinned at each other. He also thought of Matt, his best cocksucker from school. "Matt would fucking freak over these muscles," he chuckled, thinking of all the times Matt had begged Dustin to flex for him. That was old Dustin, though. Old Dustin had a nicely-built jockbod, nothing more. New Dustin blew old Dustin away. He had to do something. He couldn't stay cooped up here in his room. Not with this new body. The memory of Ty's cum came back to him again. He wanted more. More muscle. More sex. More everything.