Have you seen a doctor about your condition? Because this is quite an underweight.

There is a very rare medical condition in which no matter how much food has been consumed the person's weight remains unchanged.

You should see the doctor about your condition poor guy. Milo won't make you underweight.
Was it like that since you were born, or has it happened just recently?

If you are still growing, your weight should go up steadily.
Otherwise your weight won't change that much.
Most people have trouble losing weight; I guess you are a lucky one having difficulty gaining weight 
You just need to make sure that your body has enough energy to keep you going, then you should do fine 

hmmm, if weight aren't increasing, check if you have apetite. If your apetite is big and yet no weight gain and you gobble down a lot of food, then you might have tape worm or your metabolism could be very high.

go see doctor laa...u got problem looo...

There are no easy methods to gain or lose weight you know.
The body doesn't change simply as we wish!
An only successful method which I have witnessed is to do regular exercise and eat well.
But this can take up to 6 months to change. The body tends to "want" to get bigger and stronger when you exercise.
Your food intake will be naturally higher as well. Have you tried doing regular exercise?

2Km cycling is a good start so why don't you keep up with your cycling habits?

Certainly!! It helps a lot. Because your muscle cells need to acquire the energy to function for a greater demand through your physical activity.

Physical activity is the only direct way to help you gain muscle and body mass, but of course, this has to accompany proper diet and nutrition (e.g., good protein) :-) it makes you good looking!

metabolism could be very high

You can think of metabolism as the rate of conversion of your food into energy.
This means that if your metabolism is higher, you foods can be converted into energy faster.

Physical activity does affect and in fact increases your rate of metabolism.
As a result you are more likely to increase your uptake of food.
This significant increase in uptake of food leads to ability to cope with even greater demands (e.g., more intense activities).
As a result, your body mass will increase naturally in order to cope with eternal demands.
Therefore, your weight will increase gradually.
This is the healthiest way to gain weight    You will look beautiful !  Remember to eat vegetables okay?! 

Sounds good to me 
Rather than limiting yourself to quantity of exercise, try to do aerobic exercise:
One that makes both your respiratory and cardiovascular systems work.
So that your heart muscles and every muscle of your body can all work together!

However, the higher quality exercise belongs to the Anaerobic exercises (so-called intense training),
which helps you build endurance of your body organs and condition your body to very good shape.

My recommendation is doing basketball team sport, soccer, or things like that if you go down that road.
Trying to have some fun. Sweating indicates that your body has really done some good work!

Having said that, start slowly and don't rush!  Then you will look healthy and plump 
Hope to you getting started 

You can do it!