For his sister, he had destroyed the entire Hellhound City and offended the other Primordials. Even if his sister told him that it was because of another person with the dragon bloodline, he found it hard to believe.
It was only when his sister's guards confirmed the presence of Eren, that he started to believe about his existence. Another witness of Eren's existence was the Princess of the Nightwing Clan, who was staying in his palace.
He really wanted to find Eren, but because of the conflict with the other Primordials, he couldn't spare any time. He had activated his spies to find Eren, but even that didn't work.
It was as if Eren had disappeared like a ghost. He couldn't be found anywhere, which made him eventually drop the search, thinking that Eren might have returned where he came from.