Is he considered to have intentionally missed Fajr prayer if he doubted whether its time had expired, then became strongly certain that it had become a missed prayer and thus delayed it?

"Whoever doubts or presumes that the time for Fajr prayer or any other prayer has elapsed, should promptly perform it with the intention of performance, based on the principle that the time remains. If he prays it with the intention of making up for it (qada'), it is valid. It is not permissible to delay performing the prayer for someone who predominantly believes its time has passed; rather, he must promptly pray it even if he is certain its time has elapsed, because making up for a missed prayer is obligatory immediately, as is the view of the Hanafis and Malikis, due to the Prophet's (peace be upon him) saying: 'Whoever oversleeps or forgets a prayer, let him pray it when he remembers it.' Therefore, delaying the prayer in this situation is considered a sin for which repentance is due."