What is the ruling on the fast of someone who intended to fast the entire month of Ramadan, then it became clear that the following day was the completion of Sha'ban, and he did not renew his intention to fast Ramadan on the subsequent days?

Intention (niyyah) overnight is a condition for the validity of an obligatory fast. The meaning of "to resolve upon fasting" is to form the intention and determination for it. Articulating the intention verbally is not a requirement; merely the Muslim's determination and will to fast the next day of Ramadan is sufficient, as intention is inherent in any voluntary act. It is sufficient to renew the intention each night, or to intend to fast the entire month from the first day, as long as the fasting is not interrupted. However, if a Muslim realizes after dawn that it is a day of Ramadan and they had not intended to fast from the night, they must abstain from food and drink for the rest of the day and make up that day later.