Is the mother's payment of Zakat al-Fitr on behalf of her son for past years sufficient, even though he did not pay her back, or is he still obligated to pay it for those years?

Zakat al-Fitr is obligatory upon a Muslim for themselves and for those whose maintenance is incumbent upon them, provided they possess more than their needs for the day and night of Eid. The wife's maintenance is obligatory upon the husband, so he must pay Zakat al-Fitr on her behalf, according to the majority of scholars. If the husband is unable to pay it at the time it becomes due, then it is not obligatory upon him. Scholars have differed as to whether it then becomes obligatory upon her or not. If the husband voluntarily pays it on her behalf with her knowledge, then the obligation falls away from her; otherwise, it does not. It is preferable to pay the zakat in the place where it became due if there are poor people there.