What is the ruling on praying behind an Imam who does not pray Fajr in the mosque where he leads the worshippers in prayer?

The Imam may have a valid excuse for his delay in the Fajr prayer. Even if he does not have an excuse, his prayer is valid, and praying behind him is permissible. If abandoning prayer behind him would influence him to repent, then this is permissible, provided that the worshipper does not miss the congregational prayer in another mosque. This would be considered an act of enjoining good and forbidding evil.