Eat bigger meals lest often like 3 times a day not little meals say 6 times a day.
Dont snack.
Either go to the gym or dont go anywhere, get a playstation and lose the g/f.
Beer and pies are good.... I just dont care what they do......cant get a meat pie here anyway.
Eat lots of pasta, potato ( you want carbs ) etc.
If you have a high metabolism you are screwed as Idont think doctors will help if you are fit and healthy.... if its in your genes then suck it up.... how old are you?
Did I mention beer and pies?
JK , you can eat healthy stuff and put it on just look at what you are eating & how much how often.
Enjoy it , I was the same till I hit 30 a few years ago and could not put on an ounce no matter what.
I actually was failing my military phys cause I had 0% body fat ( the old pinch test ).
I am alot heavier now but not fat, I just filled out 6ft 1 inch 185 lbs.
I just got bigger a bit but still slim.
And trust me there was nothing I could do till then and still its the same , no matter what i eat or drink it does not help, my dad was the same..
Good luck.