You don't understand what he's going on about, as he seems to be repeating things, or chanting. But hes looking you up and down, eying you up. Again, you cover yourself, feeling self conscious, but Boy grabs your wrists and yanks them down to your sides. He doesn't stop repeating, "Boy bigger than Oom! Wrong! Oom big brother! Oom bigger than boy!" as he places his palm on your chest, just over your heart.
As his hand makes contact, your body stiffens instantly, your muscles locking up tight, unable to move, and your heart is now racing under his hand. He moves away, out of site and you hear him rummaging through something behind you. Frightened, you try to turn, but your body won't budge. You're starting to sweat with the effort, as he comes back around you, holding a dark cloth in one hand, and a crude stone knife in the other. Looking you up and down once more, he chants, "Make Oom big brother!" and slices a small X where he had his hand before, and rubs the cloth over it. You can smell the damn musky scent of sweat, dirt, male on the cloth, and the cut burns intensely. But he pulls it away, and licks up the small drops of blood. His tongue cools the wound, and he smiles. "Oom gonna be my big brother!" he beams, and then fondles his own crotch, pulling on his balls gently, and as a small drop of precum oozes, he takes it on his finger, and rubs it into your chest. "Boy gonna help!"
Your whole body tenses and relaxes as of you're working out, muscles contract and vibrate with unknown power. You hear bone cracking and your back arches in pain, your body growing a full 6 inches taller, your hands growing and pulsing, you feel your feet break and reshape. You force your neck to move slightly, looking down at your bod, which b now as gained at least 10 pounds of muscle, and an all over tan. the red X on your chest has healed fully, and you can see down your body, to your stiffening cock. Hair grows thick and heavy in your armpits and over your cock, your balls droop low in their sac, heavy with new virility. You grunt, hearing your voice drop lower, still your own, but now at the end of puberty.
You're not lanky, you're just solid, tall, masculine. You can feel the strength pulsing through your new body. You can even smell your own body's musky scent. Boy looks up admiringly and touches your cock, which springs to life immediately, and you can't help but groan, not even stopping him. He grips it tight and squeezes a few drops of pre out, then licks it up. "Oom almost done! oom gonna be my big brother soon!" he cheers enthusiastically. You're curious what he means by soon, but as the thought hits, your body tenses one final time, and every joint and muscle cracks and stretches, adding a lean bulk to your already athletic build. You cum hard and grunt almost like an animal. Your balls churning out thick ropes onto Boy's chest. He giggles and jumps up and down chanting "Oom big now! Oom big now!" and you slump over, exhausted.
He hugs you, and you instinctively wrap your arms around him, "Oom is boys biig brother now" you say in a soft yet assertive voice. You see Tarzan come in and beat his chest proudly, Boy ooks at him happily, and you just nod, knowing you are now truly part of the family.