Overall weight will depend on his position on the field. If he has hoped to be on the O-line, then he may be small. If he is a quarterback, he may be perfect.
Overall he needs an adequate balance of protein, carbs and fat in his diet. Carb-loading I typically reserve more for the endurance athletes but keeping properly fueled up is very important.
I think, in the end, good common sense is key. He is a big kid and obviously very active. He needs to eat a lot to keep up and this should be a well-balanced diet.
He should also be very aware of keeping well-hydrated, but not just water-loading the day of a game. I am sure there are college demographics for typical size for each position on the field, but at 15 the most important factor is enough of all of the vitamins, minerals and protein that his body requires to stay healthy and fit
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