There are a few reasons why your boyfriend's sheets may be turning yellow. One possibility is that he is not washing them often enough and sweat and body oils are building up on the fabric. Another possibility is that he is using a laundry detergent that doesn't contain bleach, which can cause fabrics to yellow over time. If your boyfriend's sheets are white or light-colored, this discoloration can be more noticeable.
If you're concerned about the yellowing of your boyfriend's sheets, talk to him about it and see if there's anything he can do to change his laundry habits or switch to a bleaching detergent. In most cases, the problem isn't serious and can be easily remedied. However, if the discoloration is severe or doesn't seem to be improving, it could be a sign of a more serious issue like jaundice or liver disease and medical attention may be necessary.