I'm stuck in a part in which I don't know whether to call his heterochromia a condition or not. Is there a different way to call it? I can't call it a disease or disability, for obvious reasons since it doesn't affect his eyesight or anything in anyway at all.
Depends on how the character came to have heterochromia. It may be a genetic trait, a condition cause by an external incident/accident, or the symptom/effect of disease.
It all depends on context. In a current-day setting I'd probably go for anomaly or peculiarity, but in a fantasy setting it might e.g. be percieved as a stigma.
Unless you're making a statement about the person doing the describing, I would pick something a little more neutral for a modern day setting.
He was born with it. Other than that, he pays no mind to it. Can't really say he would see it as an anomaly though.
It can be caused by disease though. I kinda like "trait". The word you choose will reflect the speaker/narrator's feelings on the matter. It doesn't bother him, but maybe he thinks it looks kinda cool, so he thinks of it as a distinctive trait. Someone else may be creeped out by it & thinks of him as the 'freak with the different coloured eyes'.
