What is the ruling for a man who has intercourse with his wife during the daytime in Ramadan out of forgetfulness?

The man does not have to offer expiation if intercourse happens out of forgetfulness. This is the opinion of the majority from the Hanafi’s, Maaliki’s, Shafa’is, and a report by Imam Ahmad that was chosen by Shaykh al-Islam ibn Taymiyyah, ibn al-Qayyim, al-San’ani, and al-Showkani.

As for the requirement of making up the day, there is a difference of opinion on the matter:
The majority of the scholars state that he does not have to make up that day. [Hanafi’s, Shafa’is, and a report by Imam Ahmad that was chosen by Shaykh al-Islam ibn Taymiyyah, ibn al-Qayyim, al-San’ani, and al-Showkani]
Maaliki’s and Hanbali’s state that he must make up the day, and that is the most cautious opinion. And Allah the Exalted knows best.