I say, and with God's success: The grandfather does not pay Zakat al-Fitr for his poor young grandson in the absence of the father or if he is poor; because the grandfather's guardianship over him is not a complete and absolute guardianship but rather limited, as it is only established on the condition of the father's absence. It resembles the guardianship of a trustee, and a trustee is not obligated to pay, so neither is the grandfather. Refer to: Bada'i' al-Sana'i' 2: 70-71, and God knows best.