My SO has an issue

EVERY NIGHT my so goes to bed he ends up soaking his side of the bed and pillow case with sweat. And I mean he SOAKS it. Like a pool of sweat. I have to change my sheets and wash them literally every day. Which I'm sure isn't good for the sheets lol. It's really disgusting though and I don't get why it happens. He isn't hot, I've asked him before. We always sleep with the ceiling fan on too. Any ideas?