If you want to increase your appetite, exercise. I find swimming in particular leaves me utterly famished. If you find that your appetite is particularly poor in the morning, this may be a sign that you're getting insufficient sleep. This is because hormones are released when you go to sleep to stop you from being hungry and promote the liberation of fat molecules from your adipose tissue to use as energy so you don't have to eat for the duration of your sleep. If you don't sleep enough, these hormones may not have had enough time to leave your system.
However, it's a little strange to suggest that not eating enough is casuing weight gain. Metabolism is a lot more complciated than you're making it out to be, but any increase in metabolism would be offset by an increase in calories consumed.
If you're worried about your weight, I would recommend removing entirely from your diet refined carbohydrates, such as sugar (esp. liquids like soft drink/juice), flour, and alcohol, and seeing if this helps.