Is it permissible for a child to call his father by his first name alone, and is that considered disobedience?

It is not befitting conduct for a child to call their father by his name or teknonym. If the father is offended by this, it is an act of disobedience (ʿuqūq). Allah Almighty has commanded that parents be addressed with reverence and respect, as in the sayings "O my father" (yā abati) and "O my mother" (yā ummāh). Addressing one's father or mother simply by their name violates proper etiquette. However, if the parents agree to it and it is customary, there is no sin. Nevertheless, the best and most proper conduct is to address parents with terms indicating veneration, such as "O my father" (yā wālidī) or "O my mother" (yā ummī).