بسم الله الرحمن الرحيم
If he does so without any Shariah uzr (excuse) merely due to negligence and carelessness and he has become habitual of it then such imam at first be convinced if he abides then it is better, but if he does not and continues missing Fajr jamat then praying salah behind him will be makrooh tahrimi.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.