I say, and with God's success: It is proper to sprinkle water on his face without slapping; so that the used water does not splash onto his clothes, thus it is preferable to avoid it. See: Tabyin al-Haqaiq 1: 6-7, Majma' al-Anhar 1: 16, and Bada'i' al-Sanai' 1: 23-24, and God knows best.