I am sorry for your son. There is no single disorder that can cause all these symptoms. If his secondary sexual characteristics developed on time ( which means he started having thicker body hair, a hoarse voice and his genitals grew in size), say around 14-16 years, then we can comfortably say its an acquired problem. An injury to the scrotum, mumps orchitis
, inflammation of the pituitary gland
caused by tuberculosis
and sarcoidosis, HIV and AIDS and illegal use of anabolic steroids can cause hypogonadism
which would lead to low testosterone
Various investigations including an ultrasound of the testes and a CT scan of the brain would be required to find out the cause.
The treatment can be done accordingly.
Hope this helps.