You could try whole milk, peanut butter or cheese, which contains protein and healthy fat to help gain weight in the right place.
Since you are growing, I wouldn't recommend any diet constraints. Try eating Six meals a day that each containing 30 grams of carbs and 15-20 grams of proteins. Green veggies are free. Fruits count. You could substitute two meals with a protein shakes made with fat free or 1% milk. Through in a banana for e,tra calories. Look into Body for Life book but stay away from steroids. My son is 14. Personally, I think you're too young to try to fight nature. Growth requires tons of calories so eat well! :)