He set it to 1 week. As a precaution, he set his mom's mind to be unable to see, feel, or interact with the Chronivac, so it would be impossible for her to undo it. He also made it impossible for her to tell anyone about the switch. He also made it so she would know how to get through her school day without any problems. He then pressed the button.
Suddenly he found himself in his mom's bedroom. He looked down and, as he hoped, saw the bulge of his breasts in the t-shirt his mom had been wearing. He heard a loud scream, a teenage boy's scream, from the other room. It worked!
He stood up and felt a bizarre shift of weight. The breasts on his chest felt so heavy, and his muscles felt so tired and weak. He felt a faint pain from his back, shoulders, and feet.
He heard loud footsteps racing toward him, and his old body burst in.
"Simon??" yelled the teenage boy.
"Who are you calling Simon? You're Simon. Are you feeling okay, Simon?" he deadpanned. The boy had a look of dumbfounded horror on his face. Wow, his face looked stupid, he thought. Then he burst out laughing, surprised at the strange sound of his mom's voice coming out of his mouth. "I'm just kidding, Mom! We switched bodies. We're going to be like this for a week."
"A week??" His mom looked down at her new body. "I'm a boy! I'm you!"
"That's right," Simon said, smiling. He was thrilled to finally be in a woman's body. "Have fun being a boy!"