I say, and with Allah's success: He is obligated to perform it, and the fasting of Ramadan suffices for the fasting of I'tikaf. If he did not perform I'tikaf, he must make up for it by fasting another consecutive month, because he committed to I'tikaf in a specific month and missed it, so he compensates for it with a consecutive fast intended for that purpose. It is not permissible to do so in another Ramadan or in another obligatory fast. Refer to: Al-Hadiyah Al-Ala'iyah, pp. 185-186, and Allah knows best.