Please explain this quote from "Blood Relations: "Brothers and sisters have I none, but that man's father is my father's son."Who is that man?

We can solve this problem algebraically with the following:

Assume you are generation 5, and 'that man' is generation 'x'.

We have: 'that man's father is my father's son'

When we have a father, we go down one generation, and when we have a son, we go up one.

Therefore,        X-1           =     5-1+1

X-1           =     5

X              =     6

Since we are generation 5, and that man is generation 6, that man is one generation later, and is therefore my son.

Or, if you're not into math:

That man's father is (my father's son)

That man's father is (me)

Therefore, that man must be my son.