He doesn't

Alan picked up the glass and took a deep swig from it, the refreshingly cool liquid coating his mouth and throat as he thirstily downed his drink. He savoured the flavour, noticing an unfamiliar and slightly salty after taste, perhaps he has some sweat on his lip, he thought to himself.

The changes began almost instantly, Alan bent over clutching his stomach as his bones shifted and his body contorted to it's new form. Large mounds appeared on his chest, swelling by the second as his waist thinned and the weight shifted to his plumping rear end. His hair grew longer and thicker as his face softened and became more feminine. His thighs thickened as his dick and balls shrivelled up inside him.

After the changes were done Alan was gone, in his place stood a smoking hot MILF. The very sight of his new father caused John to become aroused. He shifted awkwardly in his seat trying to hide his erection.

"What the fuck" Alan exclaimed in his new feminine voice

Josh suppressed a laugh as his dad moved awkwardly in his new form, it was weird seeing this smoking hot MILF move and act like his father.

"Did you do this John" his father snapped, much less intimidatingly now he was considerably shorter than John, not to mention the fact he had tits and was naked.

"Well, you said yourself no one would willingly injest my cum and I wanted to experiment and see if it worked" John replied

"Huh" Alan paused "I should be mad at you right now but this seems... I don't know... right" he mused as he cupped his new, large breasts.