Is it Islamically permissible to designate one of the father's buildings as an endowment (waqf) for his mentally disabled son, such that all its revenues go to him, while the son retains his right to inheritance after his father's death?

If a father dedicates a building as a waqf for his son while he is healthy and of sound mind, and the son takes lawful possession of it, and its ownership transfers to him during the father's lifetime, then this waqf is valid and permissible according to Sharia, especially if the aforementioned son has special needs that differ from those of his siblings.

What is prohibited is singling out one child without a legitimate Sharia reason. In this case, the heirs have no right to claim any part of the building after the father's death; rather, it exclusively belongs to the son for whom it was made a waqf.

This applies if the waqf is not restricted by the lifetime of the endower or the beneficiary. However, if it is restricted by that, it is called a "lifetime grant" (umra), and the restriction must be adhered to. The dedication of a building as a waqf does not require an official document; rather, such a document serves for authentication purposes.