Supplements
Normally, you can get from food all the nutrients, vitamins and minerals that you need. But if you’re trying to put on significant size and you’re exercising at a level that is constantly stripping your body of fuel, you’ll need help from supplements.
For calories and upping your protein intake, a protein powder is important. If you don’t want the typical gut distress that comes from eating protein powders, don’t just settle for any version of protein powder. Try to choose one without added soy and corn, with no artificial sweeteners, and with added digestive aids such as digestive enzymes and probiotics.
Other supplements that can effectively help a skinny guy gain muscle are:
-Creatine: 0.3 g/kg bodyweight for 5-7 days followed by 5 g/day.
-Carnitine: 750 mg, 2,000 mg/day, in 2 doses.
-Citrulline: 6-8 g, 30-60 minutes before exercise.
-Beta-Alanine: 2-5 g, 30-60 minutes before exercise.
None of these supplements are a substitute for lifting frequently and lifting heavy, but they will certainly help to enhance results.

Eat Frequently
Strength training is very glycolytic, which means it burns a high amount of carbohydrates. Because of this, you shouldn't go into any muscle building workout without having eaten about 300-600 calories of a carbohydrate-containing meal two to three hours prior. This meal can include sweet potatoes, yams, oatmeal, quinoa, brown rice or any other healthy carbohydrate source. If you eat another similar meal after your workout, you’ll be able to put on size more quickly, as your muscles will rapidly absorb the sugars more quickly after a workout.
In addition, timing your protein intake with several small 20-25-gram portions split up through the day will be far more effective for gaining muscle, compared to a giant protein shake at the beginning and end of the day.
Ultimately, a skinny guy trying to put on muscle should be aiming to consume a pre-workout meal, a post-workout meal and at least four to five additional meals. This may seem like a lot of eating and food, but this is what it takes to build significant muscle if you’re skinny.