What is the ruling on praying behind an Imam who does not turn his head to the right and left when performing the Taslim?

The Imam's failure to turn his head when saying the Taslim does not affect the validity of praying behind him, because turning the head during the Taslim is a Sunnah, not an obligation. Its legality has been established by the Prophet's actions (peace be upon him), and the Imam should be reminded of this.