I'm worried about his genetics. He was re-homed to me at 2yo, his previous owners did some genetic selection for him which I don't think I approve of. But he's my cat and I love him to bits and pieces and I want to know that he's ok.
Could be that he has low vision on his left eye.
You often see that with kids when they get their first set of (very thick) glasses. Before, their eyes just drift off into all kinds of directions, because they don't see anything sharp anyways.
And then you pop on the glasses, and they immediately move their eyeball into the correct direction.