absentmind

Dear scholars, As-Salamu `alaykum. One day in Ramadan, and some minutes before the call for Maghrib Prayer, I finished preparing iftar five minutes before the Prayer. Then my husband and I sat at the dining table, waiting for the Adhan. We were chatting and a while later, my husband began to eat, breaking his fast, while we were talking. Suddenly, we heard the call for the Adhan coming from the mosque next door, and we realized that my husband had absentmindedly broken his fast before the Adhan as he was absorbed in talking. Neither of us noticed that he was breaking his fast before the call for prayer. What is the juristic ruling regarding my husband’s fasting of that day? Does he have to make up for it later, or what is he supposed to do? Please reply in detail. May Allah greatly reward you on our behalf, and may His peace, mercy, and blessings be with you.