Joe,
I agree he wouldn't and shouldn't be using all that space for several years. However, also note the women's versions are significantly smaller in volume for same torso size (M). I'm assuming the women's must not have the extension collar that accounts for the 700 in^3 difference.
I like the fact this pack should grow with him. Plus, when he does outgrow it, it should fit my wife if needed. She's 17.5 so the small should still work for her.