How to gain weight for boys

1
Eat more than three meals a day. If you naturally have a really fast metabolism, eating three meals a day, no matter what’s in them, isn’t going to help you bulk up.[1] Your body burns calories fast, so you need to feed it more than it can use up right away. That means eating not just when you’re hungry, but throughout the day. Aim to eat five meals a day to gain weight.
Don’t wait until your stomach starts to growl to eat. Plan out five meals so you never have time to get hungry.
Eating this much can take a lot of effort, since you need to stock up on enough food to feed yourself more often. Pack calorie-rich snacks you can eat on the go, like bananas and peanut butter or dense granola bars.

2
Eat plenty of calories at every meal. Eating five small, low-calorie meals isn’t going to cut it; they’ve got to be big and rich in calories. Load up on a restaurant-sized meal each time, with big portions of meat, vegetables, and a carbohydrate. Eating this much might not exactly feel comfortable, but it’s the best way to gain weight fast.[2]
A large enough breakfast might consist of a three-egg omelet, two slices of bacon or sausage, a cup of roasted breakfast potatoes and a glass of orange juice.
For lunch, try a fully dressed turkey club on whole wheat bread, two bananas, and a salad.
Dinner could be a grilled steak, loaded baked potato and a few cups of grilled veggies.

3
Stick to whole foods loaded with nutrients. To gain healthy weight, eat food that’s nutrient-rich and nourishing.[3] While you could easily gain weight by drinking sugary sodas and eating large pizzas every day, this can wreck your metabolism and cause you to gain fat instead of muscle. When you’re choosing food to eat, try the following:
Look for foods that are relatively unprocessed. For example, choose old-fashioned oatmeal instead of instant, and go for fresh chicken as opposed to processed lunch meat.
Cook as many meals from scratch as you can. Avoid getting frozen dinners, fast food and snack foods, which contain a lot of salt, sugar and other fillers that aren’t nutritious.

4
Focus on protein, fats, and carbohydrates. These are the three macronutrients that will help you gain weight, and you need plenty of each of them to stay healthy. Focus on incorporating protein, fats, and carbohydrates into every meal you eat so your diet stays balanced. Here are some examples of good choices in each category:
Proteins: eggs, salmon, tuna, and other fish; pork roast, pork chops, and ham; chicken breasts and thighs; lean beef burgers and steaks; and bison.
Fats: olive oil, safflower oil, canola oil, coconut oil, and grapeseed oil; avocados, walnuts, almonds, flaxseeds.
Carbohydrates: fruits and vegetables; beans, lentils, peas; brown rice, whole grain bread, whole grain pasta, and other whole-grain products; and honey.

5
Make sure you drink plenty of water. Water will help your body process the extra protein and calories you’re taking in. Drink several glasses with every meal to avoid getting dehydrated. Since you’ll be exercising more to gain mass, aim to drink 10 glasses of water every day.
You can also drink unsweetened tea, 4 to 8 ounces of fruit juice, flavored water, and other healthy beverages.
Avoid drinking Gatorade and other sports drinks in excessive amounts, since they contain a lot of sugar.