It was his sister who first asked "Derrick?" At his nod, her eyes widened and both his parents gasped. They were looking at him like he was some sort of freak! They came running over and stared. "What happened to you!? Are you OK? Who did this!?"
While he was happy to see his family again, Derrick was getting annoyed at the attitude they were showing. "I'm fine! I like this!" And it was true. Even if some of his liking was caused by the centaur's alterations to his mind, Derrick really felt natural and normal.
He heard and felt the solid impact of hooves behind him, and did not need to turn around as he saw his family back away, staring up. His Centaur was behind him again, strong hands resting gently on Derrick's shoulders. The Centaur's rich voice said "Your son followed me to our stables, and liked grooming me. He is mostly changed now, though I thought he might like me to finish him up."
"You did this to him!?" Derricks father shouted. "Change him back!"
The Centaur snorted and leaned down to speak in Derrick's ear. "Do you wish me to finish you up or restore you to the boy you were?"
Derrick thought of the contentment and happiness he had felt, and the sensations of this more powerful body. He then looked his family in the eyes and said "This is my choice, not yours. You can be whatever you want while we are here. And I choose to have the Centaur change me."
He felt the strong hands again, this time rubbing his head and shoulders, and the sensation of change started up again. He got taller, stronger, and thicker of mind as the last changes were completed and he became a stableboy creature. When the Centaur was done, he patted Derrick's flat head and whickered "Spend time with your family. Come back to the stable when you are done."