What should a person do who is assailed by unintended sexual thoughts about his mother, and these thoughts cause him depression?

These atrocious thoughts are a deviation from the natural disposition (fitra), and it is obligatory to cast them aside and not give Satan an opportunity. Perhaps their cause is heedlessness of the remembrance of Allah and good deeds; for the heedless heart is dominated by Satan, as Allah Almighty says: ﴿And whoever turns away from the remembrance of the Most Merciful - We appoint for him a devil, and he is to him a companion.﴾ [Az-Zukhruf: 36].

You must occupy yourself with useful deeds such as memorizing the Quran and Hadith, attending gatherings of good, befriending the righteous, and increasing in supplication and seeking refuge from Satan. Know that Allah has forgiven this Ummah for what they talk to themselves about, as long as they do not act upon it or speak of it.