reciting behind the imaam

Reciting behind the Imaam

Q: I have a ghair muqallid relative. He doesant adhere to a madhab and he was saying how is my Salaah accepted if I don't recite faatiha in quiet Salaah behind the imaam. I gave him the verse of the Qur'an to prove but he said how can the imam be heaed if he is quiet. He confused me. Please clarify what should I do? I sometimes doubt.