There's no particular reason to expect him to outgrow a pack that will fit up to 19" any time soon, even in three years. I'm over 6'2", but my torso is 18.5". This is admittedly unusual, but looking at me you'd say I'm a fairly normal build for a gangly youth. I wasn't your son's size at age 8, but I think it's a safe bet to say he won't be more than a 20" torso before high school. Might not ever be.
Go down to your local sporting store and try on a small Talon 44, adjusted down as small as it can go. If it fits, this is a quality pack that will be attractive not only to your son as a 'kids pack', but could easily be worn by an adult (perhaps you, your wife, or a relative) and be a lifelong acquisition rather than a temporary one. I've used this pack, and while it's not ultralight, its very easy to use and only 2.5 lbs. Easy to pack, and more room than your son will know what to do with. If nobody in the family has a use for it, it'll have a high resale value, plus Osprey has that great guarantee.
I'm recommending what I know--I haven't experienced the packs you're suggesting, but they look like quality packs and possibly adjustable down to a smaller size than the Talon. Still, that is a full extra pound to take into consideration--while this isn't a huge factor, its got long term effects in the full lifetime's usability of the pack.