I am very thin and as i am tall i look like a bamboo stick :(?

i am 18 years old guy.i am thin from childhood.all make fun of me and i get embarrass alot because of this.i have proper nutrition everyday and almost everyday i eat fish or meat and milk everyday.what i lake still ? i do cycling everyday around 4 kms to school.i consulted with doctor and he gave me tonics n all which just increased my eagerness of eating but nothing reflected to my health.what shall i do more please help me out