Is the son's maintenance obligatory upon the father from childhood, even if the grandfather undertook it? And is the father responsible for the maintenance and accountable for it?

A financially capable father must spend on his needy children, and he is accountable before God for any shortcomings in this regard. If a father neglects to spend on his children for a period, he is not obligated to pay for the past period, unlike the case with spousal maintenance. The grandfather may demand reimbursement for what he spent on the children if his intention at the time of spending was to claim it back from the father; otherwise, not. The father must be just among his children. It is advised to find excuses for the siblings who are not to blame for the father's shortcomings.