Tefilin Retzuot are too long

My son’s new Tefilin Shel Yad are too long. When he rolls the Retzua up he can hardly close his hand. Can we cut the Retzua or does it have Kedusha? Does the cut Retzua have to be put into the Geniza? What is the halacic definition of Retzuot: tashmishei kedusha or mitzva? Thank you for your reply

The Retzuot of Tefillin are Tashmishei Kedusha .(See Shulchan aruch 154:3). If needed the Retzuot can be cut and should be placed in Geniza.