Hi Simon,
It sounds good that you care about your health. If you want to gain weight then add the food that contains fat and high-calories. But don't eat junk food.
Eat food on time.
You can try banana and milk to weight gain.

Hi,

There is one more way to try, milk and banana for some time this will be helpful to you.
Eat healthy food and dry fruits on time. Don't skip your diet plan.