According to the Sephardic opinion, he does not need to recite “Anenu” (Yalkut Yosef, citing Zera Emet, Vol. 3, Siman 62). However, among the Ashkenazic opinions, there is a major dispute regarding this. The resolution of Maran the Shevet HaLevi (Vol. 5, Siman 60) to the contradiction within the words of the Biur Halachah (Simanim 565 and 568) is that a person who does not fast due to a medical reason should not recite “Anenu”, whereas a person who ate because they were compelled (by circumstances beyond their control) or forgot may recite it. Therefore, in practice, even for Ashkenazim, a person who is exempt due to illness does not recite “Anenu”.
We note that for the sake of reciting “Anenu” aloud by the Cantor during the repetition of the Amidah, there must be at least six fasting individuals present in the minyan.