What should a worshiper do if the imam did not say "Allahu Akbar" until the worshiper had already risen from prostration to recite the Tashahhud, or if the worshiper had not yet risen from prostration until the imam had finished reciting the Tashahhud? And is the prostration of forgetfulness (sujood as-sahw) obligatory upon him?

The congregant is not required to perform the prostration of forgetfulness (sujud al-sahw) if he experiences forgetfulness while following the Imam. If the forgetfulness occurred during the last tashahhud, he should perform it and then say the salutation (taslim). If it occurred during the first tashahhud, he should stand up, following his Imam, and not preoccupy himself with the tashahhud to the detriment of following the Imam.