Hello.
Testicles are like faces: they are asymmetrical. In most men, one testicle is usually smaller than the other and hangs down lower. Think about it: if both testicles were of equal size and hung at the same height, they'd clang together. Ouch!
As long as you're not in any pain and not experiencing any symptoms, there's no problem. And contrary to myth, testicle size is not related to sperm production. It's also not related to sexual pleasure. The only time to be concerned about your semen is if/when you wish to become a parent.
However, it's important to know what's going on in your body and get accurate information, so I'd recommend that you see a urologist for an exam so that you can learn all about your penis and testicles and ask her/him your questions. Dr. J
Both testicles should be relatively even in size, but a slight size variation is normal. Some times due to some disease, a big right teticle which may occur/due to with a hydrocoele or a tumor, or if your left testicle is abnormally small which could be a result of damage from a prior illness or surgery. A hernia or varicocoele might create the impression that a testicle is larger when the swelling in the scrotum is actually not the testicle. An acute painful swelling of the testicle could be a result of a torsion and requires immediate medical attention. Since you haven't had any pain or abnormality near the area of testicles or testis bag there is nothing to worry. But if you have any pain issues then there are many possibilities, some of which require treatment, therefore, your son should see his doctor