Encontrou! É The Nowhere Boy , de Sandra Glover. Foi publicado pela primeira vez em 1997, e foi publicado na Espanha sob o nome Un chico demasiado extra .
Descrições diferentes que parecem pintar todo o quadro:
De Goodreads :
J'hon becomes part of Mandy's family as a foster child. He is unable to speak but otherwise is a genius. When Rose, a previous foster child, is in danger J'hon warns Mandy telepathically. She knows he is innocent but has to prove it when Rose is snatched.
Em Andersen Press :
Mandy is used to her parents fostering difficult children but there has never been anyone as strange as J’hon. No one knows where he has come from and he cannot speak. But he is remarkably talented - an accomplished skater, brilliant at maths and able to dismantle and reassemble a Gameboy in seconds. Has he escaped from a circus or could he be autistic? Mandy, who prides herself on her ability to get to know people, is flummoxed - but determined to solve the mystery of the enigmatic boy.
De Amazon :
Mandy is used to her parents fostering children, but following a particularly difficult experience with teenage Lucy, plus the bond with young Rose who is taken from the family and returned to her grandparents, she persuades them not to foster again. But when a strange, silent and rather beautiful boy is brought to the house in the middle of the night, social services persuade the family to take him on. But there is much more to the enigmatic J'hon than meets the eye, and when Rose is in trouble Mandy begins to hear his voice in her head and realises that he is speaking to her through telepathy.