Currently, he’s off on a hunt and gave the brothers a timeline of about a month before he returns. He’ll be seen again.
It would be hard and ill-advised to keep secrets like Oscar’s people away from him, but they could only risk it if they knew the little guys in the walls were safe. They can’t risk Oscar’s secret lightly. Considering his son is one of them, John wouldn’t be likely to hunt them.
And if he tried, Dean would stand against him.