He forgot to pay zakaat 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 Baaz (may Allah have mercy on him)
was asked about someone who forgot to pay zakaat al-fitr on time, and he
replied: 

There is no doubt that the Sunnah is to pay zakaat 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‘ Fataawa al-Shaykh Ibn Baaz,
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 Fataawa Noor ‘ala al-Darb 

And Allah knows best.