Dragon didn't talk to his own son when he saved him at Logue town..
It's a way to be suspenseful at the start of the story of bringing a mystery man who saved the Mc from harm.
But here Dragon actually knew Robin was from O'Hara and they been with each for 2 years and not just like 3 minutes compare to Dragon/Luffy. Also we are at the ending strings of One Pieces.