You are right that he is old enough to talk to doctor about intimate private issues. Thanks for commenting
At 21, if not mentally deficient your son is amply old enough to consult a doctor about such intimate issues, which may be helpful towards his self-esteem (to seek out his own solutions to such questions and problems).
A classic self-test of erectile issues is the postage stamp test (it checks to see if erections are occuring overnight).
His disease is a mixture of several serious diseases (it may be hard if not impossible to answer your question, and an answer may not be of help). Testosterone level is mostly a testicle function and is easiliy tested for by a blood test. His signs of puberty would be a clear indication his testosterone levels are present.