Is expiation obligatory for one who had intercourse with his wife during the daytime in Ramadan while not observing prayer, or is repentance sufficient for him?

Abandoning prayer is of two types: abandonment out of denial, and its perpetrator is an apostate disbeliever by the consensus of Muslims. The second type is abandonment out of laziness, while acknowledging its obligation. This person is a transgressor (fasiq) who is executed as a legal punishment, not for disbelief, and this is the view of the majority of scholars. There is also an opinion that he is a major disbeliever who has exited the fold of Islam.

Based on the view of the majority that one who abandons prayer out of laziness is not a disbeliever, you must make up the prayers you missed, and offer expiation for intercourse during the day in Ramadan, which is fasting for two consecutive months, or freeing a slave, or feeding sixty needy people.

You must engage in sincere repentance, perform many righteous deeds, and avoid bad company. Repentance wipes away what came before it and transforms bad deeds into good ones, as Allah Almighty says: "Except for those who repent, believe and do righteous work. For them Allah will replace their evil deeds with good. And ever is Allah Forgiving and Merciful." (Al-Furqan: 70).