He forgot to pay zakat al-fitr before the Eid prayer

There is no sin on him because he is excused for his forgetfulness, but he has to pay it when he remembers it. 

Shaykh ‘Abd al-‘Azeez ibn Baz (may Allah have mercy on him) was asked about someone who forgot to pay zakat al-fitr on time, and he replied: 

There is no doubt that the Sunnah is to pay zakat al-fitr before the Eid prayer, as enjoined by the Prophet (blessings and peace of Allah be upon), but there is nothing wrong with what you did; paying it after the prayer is acceptable, praise be to Allah. Even though it says in the hadeeth that it is ordinary charity, that does not necessarily mean that it is not valid and was not done appropriately. So we hope that it will be acceptable and be regarded as proper zakaah, because you did not delay it deliberately; rather you delayed it because you forgot, and Allah, may He be glorified and exalted, says in His Book (interpretation of the meaning): “Our Lord! Punish us not if we forget or fall into error” [al-Baqarah 2:286]. And it is proven that the Prophet (blessings and peace of Allah be upon him) said: “Allah, may He be glorified and exalted, says: I will do that.” So He has answered the prayer of His believing slave not to punish them if they forget or fall into error. 

End quote from Majmoo‘ Fatawa al-Shaykh Ibn Baz, 14/217 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: “Whoever pays it before the prayer, it is valid zakaah, and whoever  pays it after the prayer, it is ordinary charity”– unless the individual is excused, such as if he forgot to pay it and did not remember it until after the prayer, or he had intended to give it to the one who usually gives it on his behalf, then after that he found out that his deputy had not paid it, so he pays it himself, or if he found out that it was Eid too late, then he pays it after the prayer. If there is an excuse, there is nothing wrong with giving it after the prayer and in that case it is acceptable, because the Prophet (blessings and peace of Allah be upon him) said concerning prayer: “Whoever sleeps and misses a prayer or forgets it, let him pray it when he remembers it.” If this is the case with regard to prayer, which is the greatest of time-specific duties, then it is more appropriate in other cases. 

End quote from Fatawa Noor ‘ala al-Darb 

And Allah knows best.