What is due from someone who did not shorten his hair after Umrah until he reached Jeddah, remaining in his Ihram garments, and he did so intentionally, thinking there was no harm in it?

Shaving or shortening the hair is obligatory in Hajj and Umrah (definition). The Sunnah (definition) is for the pilgrim performing Umrah to shorten or shave their hair at Al-Marwah. However, if one shaves or shortens their hair in another place, there is no harm as long as they refrain from the prohibitions of Ihram (definition). Therefore, nothing is required of the questioner because he fulfilled the obligation, even if he deviated from the Sunnah, and he did not commit any of the prohibitions of Ihram.