What is the ruling on the fasting of a divorced person, and is it permissible to consider his fasting a Sunnah or for the sake of Allah after making up for the fasting of the oath that is due upon him?

A Muslim is prohibited from swearing excessively, as Allah Almighty says: "And guard your oaths."

Whoever broke multiple oaths and does not know their number, must offer expiation for the number they most likely believe will absolve them.

However, if the oaths were about one thing and he broke them, then only one expiation is required for all the oaths.

Your fasting with the intention that it is expiation if it is obligatory, and voluntary if it is not obligatory, does not suffice for the expiation, because the intention was not firmly for the obligation, but rather was shared or wavering between obligation and voluntary, as stated by Imam Al-Shafi'i.

If the oaths were taken before puberty, then there is nothing upon you for them, as the Prophet, peace and blessings be upon him, said: "The pen is lifted from three: from the sleeper until he awakens, from the child until he reaches puberty, and from the insane until he regains his sanity."