It is permissible for the pilgrim performing Umrah to shorten his hair and he does not have to shave it. It is recommended for him to shave his hair or shorten it at Al-Marwah Sahih Muslim 1246 a
Arabic hadith text
حَدَّثَنَا عَمْرٌو النَّاقِدُ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ، عَنْ هِشَامِ بْنِ حُجَيْرٍ، عَنْ طَاوُسٍ، قَالَ قَالَ ابْنُ عَبَّاسٍ قَالَ لِي مُعَاوِيَةُ أَعَلِمْتَ أَنِّيقَصَّرْتُ مِنْ رَأْسِ رَسُولِ اللَّهِ صلى الله عليه وسلم عِنْدَ الْمَرْوَةِ بِمِشْقَصٍ فَقُلْتُ لَهُ لاَ أَعْلَمُ هَذَا إِلاَّ حُجَّةً عَلَيْكَ .
Translation
Ibn Abbas reported that Mu'awiya had said to them
Do you know that I clipped some hair from the head of Allah's Messenger (ﷺ) at al- Marwa with the help of a clipper? I said: I do not know it except as it verdict against you.