Pointing to a man, Kalyani said, "His grandson is the only son of my brother." How is the man related to Kalyani?

Let's break down the statement made by Kalyani:
1. "The only son of my brother" $⟹$ Kalyani's nephew.
2. According to Kalyani, this person is also "His (the man's) grandson".
3. Since the child is the son of Kalyani's brother and simultaneously the grandson of the man, the man must be the father of Kalyani's brother (and consequently, the father of Kalyani).

Thus, the man is Kalyani's Father.