If You Meet the Devil, Don't Shake Hands — Sylvia Whitman

Twelve-year-old Gavin Baker, son of a warrior, is a born worrier. With his father serving overseas, Gavin assumes that he' s already imagined the worst that can happen -- until he shakes hands with his best friend Javi' s long-lost grandfather and finds himself trapped in the old magician' s ailing body. Help! As the trickster takes over Gavin' s life, fooling the girl of his dreams and even his own family, Gavin wonders if the imposter is a better version of Gavin than Gavin himself. He has to convince Javi that the real Gavin now has hairy knuckles and a love of Pablo Neruda' s poetry. Then the boys can try some tricks of their own. But will the two friends be able to reverse the old devil' s magic? Or will both of their families get their hearts broken?