Gear

some very important remarks:

Your boy/son is 11 years old, he should not carry too much weight until he has stopped with growing. Too much weight might disform him while growing when he's often outdoors hiking. This said, I'm not opossed against youngsters carrying bag packs even for often hiking outdoors.

A healthy weight is about 10% of the weight of the child.

About the good fit for a backpack you should look for the same specs of your own backpack:

The backpack should be equiped with loadlifters and stabilizer straps 

Backlength should be long enough that loadlifters are high enough abov shoulders, an angle of at least 35% should be possible. Lower the load lifters will have little to no effect and weight will be transferd too much on the shoulders.

Better a narrow but tall model than a wide and short model. Things a boy of that age might be carrying are (if it's light weight stuff), a sleepingmat (closed cell foam is very light), sleepingbag (down) and no more spare clothes than you take yourself with you when you go out (I guess some thermo underwear, an extra pair of socks and a undershort). Carrying his own drink should be possible as well.  Some quick math: backpack 1,5 kg, closed cell foam 0,5 kg, sleepingbag 1 kg, 1 liter water 1 kg and some spare clothing brings you already to a body weight of about 40 kg's. 10% is than about 4 kg carrying weight.

Some possibilities:

Bergans Knuten (don't know if it has the required straps but is suited for kids from 130 to 150)

Vaude Kimberly

If these are already too short in back length you might consider browsing back[packs of different brands in their mountain ranges. These backpacks tend to be smaller (around 40 to 50 liters) but still have enough back length for your boy.

I ordered the Deuter Climber in the end.

http://www.wildtrak.com/product.asp?ID=2626

I weighed him and he's 55Kg and I'll probably only need him to carry 4kg (sleeping bag, spare clothes, maybe an inflating mat). I figured he'd rather have a bag that looked like a regular rucksack rather than a kids daybag.

About that inflating mat, choose a short (120cm) one, it's basically enough for adults so certainly enough for kids. A short neoair is even lighter I think, but that's a rather high investment for trying out... Better start with a short second hand TAR, you can always carry that thing your self if it's too heavy.