What should the worshiper do if he has said the Takbirat al-Ihram and then the Takbir for bowing, and is about to prostrate, when the Imam says the Takbir for standing up?

The Sunnah is for thewer to join the Imam and say the opening Takbir (Takbirat al-Ihram), then prostrate without saying Takbir for prostration, or say Takbir for prostration and then follow the Imam. If the latecomer prostrates and the Imam says Takbir for rising, then the latecomer should follow him as much as possible and not prostrate.