Is it permissible for a son to prevent his father from hitting his mother, and what is the optimal course of action in this situation, and what are the mildest degrees of disobedience to parents?

If the son sees his father beating his mother severely, or for reasons other than disobedience (nushuz), he should intervene gently and kindly, in a manner that does not offend the father, advising him and clarifying the impermissibility of his action. It is permissible to seek assistance from righteous people. However, if the father is not assaulting his wife, the son should not intervene. One must be wary of disobeying parents, as it is among the gravest sins.