Is Nikah be allowed by father with his son's wife when his son has expired ? Is the Nikah halal or haram ?
It's haram "forever" before or after son's death. This is according to Quran, Surat An-Nisa' [4:23]:
Prohibited to you [for marriage] are your mothers, your daughters, your sisters, your father's sisters, your mother's sisters, your brother's daughters, your sister's daughters, your [milk] mothers who nursed you, your sisters through nursing, your wives' mothers, and your step-daughters under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you. And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allah is ever Forgiving and Merciful.
In this Ayah, The word that is used to describe "the wives of your sons" is: "حلائل" "hala-ilu" which means: the women who are (and who have been) "halal" for your sons i.e. the women who are (and who have been) wives of your sons.