It is permissible to pray behind anyone whose prayer is valid for himself.
Therefore, if the Imam of this masjid is a Muslim and doesn’t nullify his Islam, you can pray behind him. But if he believes that the Prophet salla Allahu alaihi wa sallam controls the universe or supplicates and calls the Prophet salla Allahu alaihi wa sallam or calls the saints and slaughter to their graves, this is major Shirk and you can’t pray behind him.