Salam, can a son who was raised by his Christian mother from a former marriage inherit from his Muslim dad when he dies?

All perfect praise be to Allah, The Lord of the Worlds. I testify that none is worthy of worship except Allah and that Muhammad is His slave and messenger.

If the son referred to in the question had followed and practised the religion of his Christian mother; i.e. he became a Christian and embraced her religion, then he is a non-Muslim and he is not entitled to any share in the inheritance of his Muslim father. This is because there is an impediment that prevents him from the inheritance, which is the difference of religion.

A non-Muslim son does not inherit his Muslim father according to the consensus of the Muslim scholars. The Prophet said: “A Muslim does not inherit a non-Muslim, and a non-Muslim does not inherit a Muslim." [Al-Bukhaari and Muslim]

Moreover, the Fiqh Encyclopedia reads: "A Non-Muslim does not inherit a Muslim according to the agreement of the scholars." [End of quote]

However, if that son is still on the religion of Islam, then he inherits his Muslim father. He is also ruled to be a Muslim and inherits his father if he is young and has not yet reached the age of distinction, even if he lives with his Christian mother. This is because the son follows the best of his parents in religion. The Fiqh Encyclopedia reads:

"The jurists agreed that the child follows the best of his parents in religion." [End of quote]

The same thing applies if he is young and has not yet reached the age of distinction and did not become a Christian. However, if he became Christian while he is young and has reached the age of distinction, then this constitutes apostasy from a child, according to the statement of the majority of the scholars, in which case he does not inherit.

The Fiqh Encyclopedia reads: “The jurists differed in opinion in regard to the apostasy of a child who has reached the age of distinction; Abu Yoosuf, Zufar, Ash-Shaafi’i and one of the narrations from Ahmad, are of the view that the apostasy from a child is not valid (i.e., does not count). [….] However, Abu Haneefah, Muhammad, and the Maaliki School, and the famous opinion amongst the Hanbali scholars; they held that the apostasy of the child is valid. [….] If the apostasy of the child is confirmed, then the rulings of apostasy apply therein, so he does neither inherit nor is he inherited….” [End of quote]

For more benefit that a Non-Muslim does not inherit a Muslim and vice-versa, please refer to Fataawa 358765, 238165, 137484 and 95466.

Also, for more benefit on the apostasy of a child, please refer to Fatwa 117387.

Allah Knows best.