At the end of "Marriage is a Private Affair", which change in the story's atmosphere, marked by lightening and thunder, relates to the father's change of heart toward his son's marriage?

The best answer for this question would be:

His son begs him to have a change of heart.

In the end of the story, the grandfather did not make it up to his son about their opinions on marriage. It is shown that he watched the rainfall, regretting that he had not made up with his son and that he’s missing out on his grandchildren.