Klinefelter patients have an extra X chromosome, as mentioned previously (XXY). This duplicated X chromosome promotes a pro-feminine hormonal environment.
True non-mosaic Klinefelter patients can typically have gynecomastia, lack androgenic hair developement (pubic, axillary, facial especially), and have underdeveloped male genitalia in addition to hypogonadism. They also typically have the "feminine", or gynecoid, fat distribution...very pear shaped. They may also be infertile.
They can be chimeric though, in that some of their cells contain the extra X chromosome and some are normal. These are phenotypically better off, being more in line with a true male phenotype.
To answer your question, yes, his inability to "get ripped" is most likely related to low male hormone production (assuming he is untreated). He should be on TRT, and should be followed by someone familiar with the genetic condition.