What's the punishment for someone who eats during Ramadan? What can he do to solve this problem?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If a person forgets and eats something during his fast, he is not required to do anything and his fast is valid. The Prophet said: “If anyone forgets that he is fasting and eats or drinks, he should complete his fast, as it is Allah who fed him and gave him to drink”. [Muslim] Another narration by Imam Ad-Daaraqutni relates that making up (this day) is not compulsory. However, if one eats intentionally, he has committed a great sin and violated the sacredness of this month. Also, there is a severe threat that awaits him. The Prophet said: “Whoever breaks the fast of Ramadan without having a sound reason or being ill, he could not make up for that day, even if he fasts the whole life”. [Al-Bukhari] However, Imam Ahmad, Ash-Shaafi’i, Sa’eed Ibn Al-Musayyib and Ash-Sha'bi and others are of the view that one should make up the day later, sincerely repent and seek forgiveness from Allah. Nonetheless, a group of Muslim scholars believe that expiation is required for breaking the fast of any day of Ramadan without a legal reason. They say that one should fast for two months consecutively or feed sixty poor people, in addition to making up the day later with any of both forms of expiation.

Allah knows best.