YOU SAID:

the man told his son's son that he was his grandson.

INTO JAPANESE

その男は息子の息子に、自分は孫だと告げた。

BACK INTO ENGLISH

The man told his son that he was his grandson.

INTO JAPANESE

男性は息子に、自分は孫だと告げた。

BACK INTO ENGLISH

The man told his son that he was his grandson.