What is the ruling on the prayer of one who, when led by an imam, forgot the first Tashahhud, and the congregant only realized the imam's oversight after the imam had risen from the last bowing (Ruku')? Is his prayer valid or void?

The questioner missed a rak'ah because he did not bow and catch up with the Imam in prostration. He should have performed a replacement rak'ah after the Imam said the tasleem. Since he did not do that and a long time passed, his prayer became invalid, and he must make it up.