When Jeff turned into a younger version of his dad he tried to reverse the changes but the Chronivac Beta version stoped working and he was trapped in his dad teenage body.
Unfortunatelly for Jeff, he had acquiered his dad's original body which means he also had that 2 inch long dick his dad had when Jeff first used the Chronivac while his dad kept his 6inch long dick. (Also as a 13 yo Jeffs dick was smaller, he grew up to 2 inches when he got into adulthood)
For some strange reason the awareness of the change was on and his dad freaked out when he saw Jeff as a younger version of him.
Since Jeff was now in the body of a 13yo he couldnt go to the same high school. In fact him and his dad had to fake a birth certificate so that Jeff would had a new identity and no one would ever be suspicious.
Jeff had to repeat his middle school years as a fat teenager, which made him target to bullying.
Throughout the years he had to constantly hear how much he resembled his dad. Even when he went to college.
They also noticed their bodies were connected in a strange way. Both their dicks would get hard and cum at the same time whenever one of them did. So Jeff would cum everytime his dad had sex with his new 6inch dick, and Jeff's dad would cum whenever his son masturbated his dick.