What should one who works at night do if he misses performing the Fajr prayer at its prescribed time due to being unable to perform it during work?

It is not permissible for a man to delay the prayer beyond its prescribed time due to work, for Allah has commanded the preservation of prayer in all circumstances. Therefore, he must repent and perform the prayer at its appointed times. If the work causes him to miss prayers, he must leave it and seek other employment, for "there is no good in work that distracts from prayer."