I do not care how fast your metabolism is. Keep the foods you eat high in fat. Fried chicken and so on. Potato salad is the king of weight gain. Do not waste you money on weight gainers.
Whole milk is just as good.
If you need more calories, then eat a slice of white bread every hour on the hour. Eat until you are full each sitting and try to eat more each sitting than the latter. Weigh yourself each day. Buy a freaking scale and weigh yourself. Weigh yourself in the morning and at night.