What happens if a individual tall male looses his testosterone ? his bone's get thinner and weaker at a young age

It would take a lot: to lose testosterone at 20 YO. So best to check with your doctor and discuss your underweight issues, and come up with a plan that he can monitor your progress.You may need a combination of dietary guidelines, exercise with weights starting low and maintaining tone and muscle through resistance training, and also start conditioning your body so you can do the work out easily without getting tired.

Among other things: If you lose all testosterone (medically or testicles are removed) you will lose energy, sex drive, & sexual function. You will find it more difficult to keep weight off and over years your bones will lose density. It's also harder to maintain muscle mass. You can decrease risks by eating right and maintaining exercise.