In the room where he's meant to transform he sees a giant machine, it's black, somewhat biological looking and with holes for humans to step into. A sign on the wall reads "machine for genderless conversion. To remove the sex characteristics of any consenting adults. Please remove all clothing and enter".
There are two other people getting in. A young girl who looks about his age, and a woman a few years older who currently has a penis. He tries not to stare as they remove their clothing, their elegent naked bodies contrasting the coldness of the machine.
He looked at the girl around his age. He wished he had a chance to talk to her. His body turned on just from seeing her. He wanted to feel his penis inside of her, but he remembered that soon he wouldn't have a penis, and she wouldn't have a vagina. This is what he wanted right, at least what he needed. He'd be giving up any chance to be inside a girl for awhile though, shame there was no option for a last chance now.
As he took off his clothing he looked at his penis, it was a nice penis, it would be a bit sad to say goodbye to it for awhile, there were things he would miss a lot about being a man. But it was just a vacation. If he really missed it he would have a back in six months if he wanted to. Still six months would be a long time. He really took time to feel his penis, his balls, his body hair, before they were gone, and than steps in.
He walks into the machine naked, and if closed on him. It's entirely dark. He can't see anything, it latches something onto his body as he's made to lie down in a coffin like chamber. Things inject into his body as needles attach to him. His penis is made to harden, and than placed snuggly into some sort of tube as something else holds onto his balls...