Small penis Hasnt grown (15 soon 16yrs)

hi im 15 soon 16 years old and my penis hasnt start growing. my penis is 4cm/1,5 inches when flacid and 9 cm/3,5 inches (also very thin) when erect and im afraid that my penis would not grow. my brothers penis is 16 cm (he told me) when erect and his penis started growing when he was 13 i am very afraid that my penis wont grow and then i will never get married im also talking to a girl that i really like but she is turned on at times and im afraid to show her my penis and plus i get boner almost all the time and i ejakulate fast. i have a dark voice har under my armpits hair on my balls and over and it comes cum when i masturbate, 178cm tall but the only thing thats left is beard my my penis to grow. please help i would apriciate it thanks