Jeff thought about what a parasite would want, and also what he would want and the ideas began to come to him.
A creature acting like an organ would want to trick the body into giving it blood and nourishment, and Jeff wanted this too so that when it is left as a normal organ it can survive and keep working for him. It would want to protect Jeff from infection from other sources and speed recovery of any injury so there wouldn't be any interruption in the food supply. It would want to be able to release access fat stores to help it grow. It would want to make its host better than average somehow, so that the host won't try or even want to remove it.
Jeff thought about how this new symbiotic gland could make him better and began to smirk. It will simply push the host body toward its prime and then keep it there. Obviously a 70 year old won't look 25 but could be made more youthful. Jeff still had more growing to do, so being pushed into his prime would require fuel.
Jeff set up the machine to turn his dad into something that would act like an organ in his own body. His body would accept it and once his dad had grown, it will start modifying his hormones and working with his immune system to make his body a safe haven for the thing his dad has become. Jeff thought about how to keep his growth from making him need to eat and sleep all the time and began typing away. The parasite is feeding off of Jeff but after it begins modifying his body, if it needs more nutrients, it will pull them from his father's original body, we will see what that means when he reverts back to human.
Jeff thought that was sweet sweet irony... teach his dad to force his hand... As for sleep Jeff decided that when the organ is changing his hormones it will intensify his sleep and make it more effective. He will eventually only need 3 hours of sleep and that can be broken into 1 hour increments throughout the day.
Satisfied, Jeff accepted the modifications. Inside his body, a tiny tapeworm transformed and decided it needed a different location to make its home. It gnawed a tiny hole out of Jeff's intestine and secreted a mucous to keep anything else from coming out with it, to protect its host. It crawled around and eventually found a large vein to connect to. It began releasing chemicals to fool the body into thinking it's blood supply had been injured and surrounding arteries began sending tendrils of nutrient rich oxygenated blood toward the rice grain sized organism. Until full integration however, the new organism made a tiny incision on the vein and took nutrients from Jeff so it could grow. Several hours later the creature had quadrupled in size and had begun to wrap around the vein completely. It made another incision in the vein but this time to insert chemicals into the blood.
By the next day the creature/organ had begun its slow takeover of Jeff's body. From it's perspective, making it's host more active, and athletic will make it require more food, which would mean more nutrients in the blood, which it will soon get direct access to from the arteries...
Jeff awoke the next morning feeling weird... and hungry... he noticed his face looked just a little different but couldn't place how. The creature had spent all night signalling to release all fat stores into the blood stream and excess fat helped the creature to grow. It was now the size of a small grape, continuing its explosive growth speed.