to gain those extra calories you may want to invest in a good weight gainer shake...that can easily get you about 1500-2000 calories by themselves..eat calorie rich food high in protein and carbs...just watch the saturated fats and avoid alot of sugary products...oats, wheat, potatoes, pasta, meats, veggies, rice, cereals... here is a decent article
High Calorie Food - the secret to packing on muscle