Is the reaction of fear, trembling, and crying, followed by praise, seeking solace in God, a prayer of gratitude, and supplication, to the sudden death of a son considered a sign of impatience, or will its owner be rewarded for it?

If a slave's child dies, and the slave is asked about his condition and he praises Allah and says "Innallahi wa inna ilayhi raji'oon" (Verily we belong to Allah, and verily to Him do we return), Allah will build for him a house in Paradise called Bait al-Hamd (The House of Praise).

And a Muslim who is afflicted by a calamity and says, "Innallahi wa inna ilayhi raji'oon, Allahumma ajirni fi musibati wa akhlif li khayran minha" (Verily we belong to Allah, and verily to Him do we return. O Allah, reward me in my calamity and compensate me with something better than it), Allah will compensate him with something better than it.

And complete patience is shown at the initial shock, due to the great reward that results from it.

Even if one misses the reward of complete patience, he is not deprived of the reward of those who are patient, as mentioned in the texts.