Dear Castello,
You are as a young male growing and developing. When I was in high school I was 5'8" tall and I weighed about 125 pounds on a good day. I am 56 years old right now, so in the time I grew up and was your age I was abused because medical science and bulletins to gym teachers tried to induce conformity. I was also the "skinny" guy in my athletics and I paid for it there too. The only thing I had that was different than most was a great deal of body hair for my age. Even that was far from through maturing.
As I aged everything changed. I was when I was your age unable to easily even develop any muscle. I can tell you that I tried every protein powder and weight gain preparation made. In the late 1960's and early 1970's they were all horrible! I never did at that time gain weight or muscle bulk like I wanted.
I was very lucky. My Stepfather was an MD and a Surgeon. When he found out what was going on and the pressure I had from a couple of athletics coaches, he became very angry. He did not make a telephone call, he personally called a meeting with my H.S. Principal and proceeded after identifying himself as a Medical Doctor to first rip a new ass in two of the coaches and then in the Principal. He told them that the only way I was going to gain weight at that time was to use steroids and that at my age other damage would occur if that happened that could be permanent.
After High School and in College my weight and muscle mass slowly began to change because my metabolism was changing to allow weight gain and a gain in muscle mass. I was again 125 pounds at 18 years old. By the time I reached my 23rd birthday I had made it to 150 pounds. By that time weight training and other things started to do some good. At 56 years old I am now a big hairy bear kind of guy and I am not a fat man either. At 5'9" tall my body weight is now about 185 pounds. No, I do not have a 29 inch waist, but, I exercise, I ride horseback as much as I can (an excellent core strength builder by the way) and I am physically very active around our ranch.
You are NOT too skinny. Being skinny now means that you are going to be far less likely to have a severe weight problem as you age because as your metabolism changes with age, you will be less likely to pile on the pounds in an unhealthy way.
Now, here is the final good thought for you.
I do not know how familiar you are with American actors. Do you know who Ashton Kutcher is? Do you think that Ashton Kutcher is to skinny? I am retired from the movie and television business. I have seen Ashton Kutcher in a pair of swim trunks in person. I can tell you that your general build is just great as it is and it will mature and fill out with a little more muscle bulk as you age.
Take care of yourself, eat right, as your body will allow do some weight training and the muscle mass will happen for you in time. Right now you could be a model without any problem. You have the general build that most male models have and the one that is desired.
My young friend, you definitely have nothing to be ashamed of in any department! You are a good looking guy that many would fight for without hesitation.
