ANSWER:

You need to talk to him. If he is unable to pronounce it correctly, his prayer is valid but you shouldn’t pray behind him due to corruption in his Aqeedah. But if he can say it correctly but doesn’t want to, the prayer is invalid. 

There is a deobandi masjid where I happen to pray. Sometimes the Imam say ‘Allahu Aghbar’ instead of ‘Allahu Akbar’ in takbirs other than the opening one. But he doesn’t perform sujood as sahw at the end of prayer or even after Salam. Now do I have to repeat the prayers behind him (though he didn’t make mistake in pillars)?

Prayer, Sects | March 12, 2019 6:59 am
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Can I pray the Sunnah after the Fard Prayer without any valid reason? – Sheikh Assim Al Hakeem
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How should a female prostrate in salah?Is it same as men do? Because where I live, male and female do slightly different Prostration as in a female doesn’t lift her elbows .so there are some differences in a male and a female prostration.I want to know the most correct way of doing sujood.