His ablution is valid, but he would be neglecting a virtue by not starting with the right side in the ablution, which is recommended. The one who performs what is recommended is rewarded, and the one who neglects it is not blamed. See: Al-Tanqih 1: 124, Sharh Al-Wiqayah p. 84-85, Al-Hidayah 1: 13, and Al-Binayah 1: 187.