Site won't let me post a second answer but I just thought what it could be. Does it look like the link below? Cats have a 3rd eyelid that is normally retracted into the corner of their eyes and can't be seen. When they are sick it can come out/stay out when they have their eyes open. It can also show when they are asleep but have their eyes open even when they are healthy.
If you can see teh 3rd eyelid on your cat all the time then taht means he's sick and needs to see a vet.