Don't think 100 grams a day of protein a day would hurt the kid. He is probably getting less than half that now if you analyzed it. I think that is a crucial age to be getting the protein too. That is a lot of the reason why kids are so scrawny in their upper teens. Ever notice the families that eat the gargantuin amounts of meat at every meal grow the big kids? But often eat too much of everything else though too & end up overweight. So yeah, he may need a little more complex carbs etc, but I agree, watch that, he doesn't need the fat. as long as he has enough quality sugar to get the energy to practice & work out.
You got to look closly at his protein intake, no way 100 grams could stress the liver, just make sure he drinks plenty of liquids too. And yeah, I agree he needs a good basic weight program. I bet something like a typical 3 way split, just working out 3 times a week about 20 minutes each time would be enough. But both the diet and the lifting would have to become a week-in, week-out habit, every week consistantly.
One or 2 whey shakes would take care of all the extra protein he needs, added into his regular diet.