If he says the Takbir while he is closer to standing than to bowing, his initiation of the prayer is valid, even if he intended it as the Takbir for bowing, and his intention is disregarded; because someone who catches the Imam in bowing does not need to say the Takbir twice. However, if he is closer to bowing than to standing, his initiation is not valid; because for the Takbir to be valid, it must be said while standing or slightly bent before he is closer to bowing, and here the condition is not met. See: Maraqi al-Falah p.217.