Is it obligatory to continue to forbid grandfather from increasing the number of rak'ahs in prayer, even though he believes he is right and others are wrong?

If a person becomes old but retains their awareness and intellect, then prayer does not cease to be obligatory for them, because intellect is the basis of religious accountability (taklif). If they pray alone, then what matters is their own conviction, and Allah will overlook their mistakes if they erred after having exerted their utmost effort. It is preferable to help them perform congregational prayer in the mosque if that does not cause them hardship, so that they may follow the Imam. However, if senility overtakes them and their mind becomes confused, then prayer is not obligatory for them, and they will not be held accountable for any deficiency in it.