I'm not claiming to be an expert, but will share what my son is doing in case it might help anyone. First, I can't recommend highly enough the book "Building the 95 MPH Body" by Ben Brewster. It's very affordable and has a wealth of information regarding diet and strength training. I bought it for the weightlifting content and didn't really pay that much attention to the nutritional section until just recently when my son got on a steady lifting program with his high school team. FWIW, my mom's former 40 year career was as a Registered Dietitian. I asked her to review the material and she agrees all of the information is solid. The information is presented in a manner that is thorough yet easy for someone with no background in counting calories to follow and understand.
The bottom line is this: You can't just guess at how many calories you're eating. You can drink protein shakes and take weight gain pills but if your caloric intake is not more than your caloric expenditure you will not gain weight. I used the material in the book to identify my sons needs for calories, protein, carbs, and fats. I downloaded the free program MyFitnessPal (google it) and we started tracking. I admit it was a bit overwhelming for the first week, but very quickly it became easily manageable. I still help with the tricky stuff like when my wife makes a casserole and we need to know nutritional info/serving, but my son does 90% of the food logging himself. I like it that it's time I don't have to spend doing it plus my son is invested in the process. He has gained 7-8 lbs in 12 weeks, which is less than he was hoping for but not bad at all. I attribute some of that to the 3-4 weeks it took to really dial in what his caloric intake needed to be, and some to the fact that his PE class is basketball, so he spends an hour a day running up and down the court. That's great for cardio and agility, but counterproductive to explosiveness and weight gain. He is agreeable to adhering to the plan because the #1 directive is always "eat more!" and what 16yo boy doesn't like that kind of diet. He eats healthily, but there is really nothing that he can absolutely never have so he doesn't feel like he's sacrificing too much. He eats lots of fruits, cereals, bagels, pastas, breads, almonds, power bars, turkey sandwiches, etc... and is fine to throw in a milkshake or bowl of ice cream here and there to meet the calorie count. And then there are just cheat days. If the team goes to Hooters after the tournament, he eats his 20 wings like the rest of the boys. FYI - He is 5' 10" 175lbs. His target daily caloric intake is 5500.
Some takeaways after doing this for a few months:
-Everybody who's trying to gain weight or muscle mass drinks protein shakes. I found out an interesting thing when we started tracking my son's intakes. He easily meets and almost always exceeds his RDA of protein with the food he eats. I don't think I'm making a controversial statement when I say if you can get the nutrients from non-processed foods, it is better than getting it from supplements. He also easily meets his fat content daily, getting good fats from dairy, eggs, almonds, avocados, etc... The only thing he struggles to meet are the carb requirements.
-If you need to eat a lot of healthy calories invest in a Nutribullet personal blender. One of his staple smoothies uses a mango, banana, avocado, tbsp honey, 1 cup of milk, 1/2 cup vanilla yogurt, and some ice cubes. 3 minutes to make and there is one very healthy 750 calorie snack.
-You have to be dedicated to do this, but we don't go nuts. If he spends the night at a friend's or we have a long day of tournament baseball, we just have days he doesn't track his food. He's learned so much about what he's putting in his body he gets pretty close anyway. Again, when in doubt, eat more.
