Adding 20 Pounds by Summer Practice

I don't necessarily think there is anything wrong with gaining 20 lbs in 6 months (that's only 3.33 lbs per month) and it is very feasible considering the amount of testosterone that is running naturally in a 15 year-old's body. 

However, my question is 20 lbs of WHAT???  Fat?  Muscle?  One will lead to a DECREASE in performance, while the other will lead to an INCREASE in performance (assuming the athlete already has some skill at the position). 

You said the coach is dead set on him gaining 20 lbs, however if that 20 lbs comes from fast food, little debbies, and snickers bars...your coach just created a slower, weaker, and less powerful athlete...regardless of how much the athlete lifts during this timeframe.

On the other hand, if the weight gained from lean protein sources, clean carbohydrates, and a higher amount of quality (mono and poly) fats, then he just created a stronger, faster, and MORE powerful athlete.  Even with extra weight, as long as it is in the form of lean muscle gain, he can become faster and more powerful as his relative strength (strength in comparison to bodyweight) will have increased.

What I would recommend is not putting a single "number" on the amount of lbs the athlete should gain.  Doing this can be counterproductive, as mentioned above.  I would just increase the athlete's Calorie intake (by way of added carbohydrates) and closely monitor his relative body strength during the "gaining" phase.

In other words, have him eat more and lift hard, while keeping close tabs on his performance in Chin Ups and Standing Broad Jump.  If either of these goes DOWN during the weight gain...STOP and re-evaluate his diet plan, because it means he is gaining extra FAT MASS.  This is not good and will NOT help him become a better athlete in ANY WAY.   He might be bigger, but he will be slower and less athletic because of the extra fat mass.

Be careful about meal timing as well.  On a non-lifting day, you might want to lighten up on the carbs just a bit.  On lifting days, absolutely go it with clean carbs. 

Keep his protein intake to a minimum of 1 gram per lb of bodyweight (up to 1.5 per lb. of bodyweight)

Keep his fat intake at approximately 25 - 35% of his total Calorie intake (2/3 of this number should come from mono and polyunsaturated fats - the other 1/3 from animal fat)

Increase his carbohydrate intake by way of wheat pastas, rice, potatoes, bananas, oatmeal, grits, beans, and any other clean carbohydrates that are more dense in calories. About 2.5 - 3 grams of carbs per lb of bodyweight for a minimum. 

Also, the weight gain shake mentioned in another coach's post is great for packing on lbs. 

Hope this helps some.

Coach Hale