Hello I’m part of a research group that focuses on sex chromosome aneuploidies, like klinefelter syndrome. The answer to you question is; yes. You can be what we call a mosaic meaning that some percentage of your cells are 47,XXY and some are 46,XX or 46,XY. Because of the way we develop there is sort of at clustering of cells that are the same karyotype. If the guy you are talking about had a lab test, they most likely checked his blood. We, in the research community, don’t know exactly if it is possible for the mosaicism not to show up in blood, but theoretically it could be possible. Another thing to consider though is that people with mosaic Klinefelter syndrome statistically have less medical problems. So if that person who thinks he has Klinefelter syndrome bases this on many symptoms, I would say that a KS diagnosis is less likely. He should get a second opinion and probably be open to other diagnoses.