What is the ruling on a father's non-adherence to prayer in the mosque and his delay in Fajr prayer, and how can he be advised?

Remind the father gently of the importance of congregational prayer in the mosque and its virtues, while taking the means to wake up for prayer. If he prays in congregation at home, there is no sin upon him, but he misses the virtue of the mosque. He can be reminded through indirect methods, such as lectures and books.