"This had been happening for a long time and this kept him from marrying anyone. I don't know whether it was a curse or what." How about these two sentences?
You have already expressed the idea that early death and a father's death coinciding with the year of his son's tenth birthday throughout the history of his family (you used "his entire family") so saying "This had been happening for a long time" is redundant. You can delete that completely and delete the word "and".
The history did not actually keep him (prevent him) from getting married. He allowed the history to deter him from marrying. However, there is a lack of logic here. It does not say that all the men in the family were married. It could be that the single men died before the age of 50 too, so it would be perfectly reasonable for him to assume that he would die before the age of 50 whether he marries or not. If it's only the married male ancestors who died at an early age, you need to make that clear earlier in the piece. I don't see why he would deny himself the happiness of getting married just because he thinks he'll die before he's 50. The history of the son's tenth birthday coinciding with the death of the father is also irrelevant really. He could get married but not have children, thus avoiding the risk of it happening to his son. However, the documentary clearly suggested that he allowed these things to colour his view on marriage.
So here's what I'd write: "This put him off marrying. The history of the deaths of the men in his family is very curious/mysterious - maybe it's a curse!"
Don't use "I don't know if it's a curse
or what" - using "or what" at the end of a sentence is very casual/informal/colloquial.