Is a son's refusal to attend the division of inheritance, when asked to do so by his father, considered disobedience, knowing that the father has cut ties with his family and has not attended any previous divisions?

You must obey your father regarding attendance at the inheritance distribution if no real harm will befall you. If your father is distressed by your refusal to attend, this falls under forbidden disobedience to parents. Part of honoring parents is to enjoin good upon them and forbid evil gently and courteously, without violence or abuse. Address your father with reverence and humility, for honoring parents is among the greatest means of attaining Allah's pleasure and entering Paradise.