Thank you for providing quick additional info.
Loss of sex drive can be due to various causes: suboptimal amounts of testosterone
in the body,stress and anxiety,depression,too much alcohol consumption,some medical conditions like cardiovascular disease,diabetes,certain medications like anti depressants,changes in hormone levels.
The severe pain you have described in your boy friend's left testicle can be due to Infection or inflammation like epididymitis(inflammation of the epididymitis-a duct through which sperm leaves the testicle) or orchitis
(inflammation of a testicle with bacteria or a virus).
This pain does not seem to be due to prostate infection & I think he needs to get it checked at Emergency Room(ER).He needs ultrasound of the testicles,complete urine examination and other tests.
So for confirmation,I advice your boyfriend to consult doctor at ER to have these tests done so as to know the exact underlying cause & then treatment will be done accordingly.
Hope this helped.
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Thank you *****