My son had a sports physical and the doctor said, your testes are smaller than other kids your age.  He is 17.  My son did not ask, and this doctor in my opinion was very wrong to say this unless sure.  Can doctors tell very quickly if testes are so small that need testng
Hittites size varies among individuals. It is not the size but whether it functions properly or not that matters. The testis produces sperms as well as hormones. Hormone function is tested by blood tests and sperm function by semen analysis. Asking your son to get these tests done might be too traumatic mentally for him if he has not got any symptoms.Hence, I would not advise you these tests now. If you are still worried then you can get an ultrasound of the testis done that will rule out any major problem with testicles but not confirm whether it is a small testicle or not.