Im a 100lbs 5'5 20 year old male

Why the h*** can't I gain weight,I have the fastest metabolism ever.I eat normal like anyone else 3 meals a day plus 3 snacks.I also look young as h***,the only grown looking thing about me is my 7 inch p****.I don't have facial hair and I still get pimples like Im a damn teenager.Help me what can I do to get bigger?