Is the funeral prayer obligatory for a Muslim who died more than 15 years ago and whose family did not pray over him due to an oversight?

If the deceased is buried without a funeral prayer, it is permissible to pray over his grave for up to a month, according to the majority of scholars, based on the hadith (definition) narrated by Al-Tirmidhi from Saeed ibn Al-Musayyab that the Prophet, peace and blessings be upon him, prayed over Umm Sa’d a month after her death. However, if more than a month has passed, praying over him is not prescribed.