Eatting more meals (4-5) times a day might be a bad idea for gaining weight. You might to keep it at 2-3 full meals daily.
When I was your age, I couldn't gain anything. I ate really fatty food, eat right before I sleep, drank soda like water. I even took protein drinks and gained zip after a 6 month try. Now that I'm 30, I want to lose weight.
My suggestion is to take protein drinks (along with working out) and see if it helps you.