I say, and with God's success: The father pays Zakat al-Fitr for his adult insane son if he is poor; because his maintenance is obligatory on the father, and the father's guardianship over him is complete. In contrast, if his adult insane son is wealthy, he does not pay for him, but rather it should be paid from his own wealth; because he is wealthy. Although the father has complete guardianship over him, his maintenance is not obligatory. Refer to: Bada'i al-Sana'i 2: 70-71, Al-Wiqayah p. 230, Al-Hadiyyah al-Ala'iyyah p. 213, and God knows best.