Why is my boyfriend’s side of the bedding so discoloured?

Hi all,

So, over the past 2 years I have notice that my partner’s side of the bed is all yellowed. His pillow, his side of the duvet cover, his side of the bed covers. And this has happened to multiple sets as well.

Edit: we live in an extremely humid country in Asia. And he is a Nordic. Not sure if it gives some additional context.