I went through and am going through the same thing. Some people just develop later than others in life, you'll fill out eventually. What I did is learn to fight. If you are undersized growing up highschool becomes a living hell and you will run into people looking to bully and humiliate you. I took up boxing and karate and started weightlifting. After I pounded the shit out of one guy who was picking on me since 5th grade ppl stopped fucking with me, even though I was undersized. Learn to fight and scrap anyone that fucks with you. Don't take shit from anyone, no matter how bigger they are.