Does the son inherit the entire inheritance after the father's death, or do the father's brother, sister, or other محارم inherit with him? And if there is no son, who inherits the father?

The rule in inheritance is that the male inheriting descendant (son - son's son, no matter how far down) completely disinherits all collateral relatives. If someone dies and leaves no heirs except a son, then all the wealth goes to the son by virtue of 'asaba (agnatic relationship). If someone dies and leaves no heirs at all, their wealth goes to the Muslim treasury (Bayt al-Mal). It is advisable to refer inheritance issues to the Sharia court or consult with scholars directly.