Majority of scholars say that if he had a legitimate reason for not being able to make them up in that time frame, then nothing should be done.
Majority of the scholars say, that if had no legitimate reason to not make them up, then one of the following should be done:
His heirs should make up the days. This is based on the following hadith:
‘A`isha, may Allah be pleased with her, narrated that the Prophet (pbuh) said, “Whoever dies with unfulfilled fast days, a relative is to fast on his behalf” [Bukhari and Muslim]
or money should be taken out of his inheritance before it is distributed and should be used to pay his kaffara, which is to feed one poor Muslim person per missed day.