He spares the son of Arjuna for he is his pupil. Pupils, their sons and grandsons, are ever dear to the righteous people.
अर्जुनस्य सुतं त्वेष शिष्यत्वादभिरक्षति। शिष्याः पुत्राश्च दयितास्तदपत्यं च धर्मिणाम्॥