.... shrink.
Nothing John would notice at this point, but Kevin knew what he needed to do now.
"Sit on it, John."
John looks up at his master and doesn't even think twice about it. He starts to straddle Kevin's dick immediately, but Kevin stops him.
"Face me."
John gets up off of Kevin and turns around to face him. It might just be the angle, but John thinks that Kevin looks bigger than he did before. Before he can spend a whole lot of time trying to understand this, Kevin's dick is at his hole.
John's surprised that it is hard again, and that it feels so big. Then again, nothing that's happened today really makes any sense to him. Then, just to add to his confusion, before he can start to try to sit down on Kevin's dick, it starts sliding up into him on its own.
He looks up at Kevin's face, who is smirking at him.
"Oh I think he likes you, John. Don't worry - you two will get better acquainted shortly."
And with that Kevin bucks his hips and John loses his balance. It's only then that John realizes that he didn't fall over, and that Kevin's dick supported him.
Then John noticed that his legs were gone. In his place, he saw that at his waist, he melted into Kevin!