Yes, if the call to prayer was during his witr, he should complete the prayer and he is excused. This matter is part of the case of the timing of witr prayer: i.e., does witr end at the start of the timing of the fajr prayer or at the end of the timing of the fajr prayer? The majority of scholars (al-jumhoor) are in agreement that answer is the start of fajr prayer.1
From the book What Should You Do in the Following Situations... ?
1 Is'aaf Ahl il-Asr bi-ma Warada fi Ahkaam Salaat il-Witr, by Faihan al-Mutairi, p.33.