Some boys/men have a smaller left (usually) testicle due to varicocoele (poorer venous drainage from testicle). If it is due to varicocoele the scrotum looks a bit enlarged above the testicle with the person in a standing position. This is much like varicose veins in the legs. This can be due to a normal anatomic variation. See a urologist if your son is worried. Varicocoele can be surgically treated if warranted.