Abolfazl Sabouri, Abolfazl Sabouri is based in New Zealand and has an MA in Jurisprudence and Islamic Studies. He is a graduate of Elmiyeh seminary in Qom with more than 15 years of study and research where he has... Answer updated 3 years ago

If he used to forget the pillars of Salah, he can not stay as Imam. However if you mean by pillar, like Tashahod, Salam, Qonut, Zikr and Reciting after Fatiha, his Salah is valid, he can do sajdat al-sahw when remembr. But if he forgot pillars like Roku, two Sajda or Takbir, that Salah is not valid.