For what it's worth, the main part of my pack is 40 liters, and even with a 10-day food supply I've never used any of the extension collar. I am a lightweight backpacker but definitely not ultralight.

As mentioned, you should box up your gear and gather the equivalent in weight and bulk of a week's food and a day's water before you head for the store. (Make an appointment with the store and ask for their most experienced fitter.) The pack needs to fit your gear and fit you with your gear inside.

All those recommendations are pure guesswork as they have no way of knowing what you carry. (Nor do we, for that matter.)

Don't expect your pack to fit your son any more than you'd expect your shoes to fit him. Pack fit is almost as individual as shoe fit.
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey