"That is too low an amount of calories for his energy expenditure - in athletes this can lead to the equivalent of the female athlete triad, but for males - so hypogonadism, disordered eating, and low bone density (not sure if that has been checked).  Regardless, the hypothalamus is likely the culprit for his low testosterone levels, and it will be much more likely to shut down in the face of this type of energy mismatch."

He needs many calories to meet this growth demand (with or without ED). Feed him high calorie, high fat, low volume foods at every meal and snack...The ED thoughts in my sons head were so powerful I decided to not leave him alone with his thoughts for 20 months. I made sure his mind was occupied...