Which Gun to Buy for Son?

I am looking at buying an air rifle for my 12 year old son for Christmas. He's been killing chipmunks and squirrels around our house for years with his 10 pump and he's wants something a little more powerful to be able to hit them when they are way up in a tree. I don't know much about air rifles so I am looking for some help here. I really don't want to buy a youth model but I'd like something smaller and easier for him to use. Can I get some suggestions on what would be a decent rifle for him? Thank you.
 
If you want a good springer, take JoeWayneRhea's words to heart, he knows his springers. If you are considering a PCP, the best gun for the smallest outlay of cash in my opinion would be the .25 Marauder. Now with that said I will say this right up front, it would be a great gun for your son to grow into, and especially if he has an interest in airguns for the long term. Now the bad, with the PCP you will need a way of filling the PCP. Personally I started with a Hill MK3 hand pump, still got it. Not a bad way to go and it keeps you engaged in the sport and the importance of choosing your shots. (since you have to pump) Accuracy is great, power is great, ability to grow into....Heck I'm in my 50's and still loving my Benjamin Maruader .25. Good luck with your choices and Merry Christmas to you and your family.
 
Diana RWS Model 34 or 340 N-Tech or similar. They are a fine general purpose rifle and the price is reasonable. They are neither purely target oriented nor purely hunting oriented. One in .177 will take squirrels and rabbits humanely out to 40 or 50 yards, if you do your part. The rifle will do it's part out of the box. Most Model 34s will hold a dime sized group at 25 yards with the right ammo. Typically they like a nice middle weight pellet and shoot at about 14 foot pounds of energy. A synthetic stock model will make a fine young hunter's rifle.
 
I think this requires a bit more information to give The Best Answer.

a) What's your budget?
b) How big (height) and strong (upper body) is he?
c) Do you live in a noise-sensitive area, or is it OK if it's a bit louder than the 10-pumper he's got?
d) Where do you live? There's always the chance there's someone local (or local enough!) to help you out in person. For example it would be very useful to have your son along "just to shoot with a buddy for a day" and see if he can cock a medium-powered springer reasonably.

Benjamin sells a Discovery kit, that includes the pump. Around $300-400, I think. Sonny Boy might need Dad's help to pump it up, depending his weight. (my boy is 12, and weighs about 87 pounds; he can't pump up my Marauder) But that's not necessarily a bad thing. He'd have 20 shots or so before he comes back to you, and they would be shots with no recoil or hold-sensitivity. If your area is noise-sensitive, you can buy a moderator for it that will quiet it down a lot.

Alternately, my .177 Marauder is currently for sale. A nice package, and a nice step up from a Discovery. It's been for sale for a few days now with no serious bites, so if you want the gun without the scope or something to save a few bucks, we can deal. It shoots at 18 foot-pounds of energy, which is about triple what he's got now. It will handily take down a squirrel at 35+ yards. You'd just need a pump and you'd be ready to go with that. 

Here's a video review I did on the gun, before I decided to sell it.

If it's out of the budget, there are plenty of good options at lower budgets too. The only question is whether he's strong enough to cock them. The aforementioned HW30s or Beeman R7 is a great gun, but not any more powerful than his 10 pumper. (thought it may be a bit more accurate) My son can barely cock it; I don't think he could cock the next step up, in a springer, which is the HW50. So to me, that points to a pre-charged pneumatic (PCP) air rifle as being the next logical step.

Or possibly a Benjamin 392, which is a higher grade pumper, .22 caliber, and only needs 8 pumps. (but harder ones) Again, upper body strength may come into play here, depending on how sturdy he is. It would be invaluable if he could try some guns out from a local member. I'm in the north suburbs of Chicago, and I'd be happy to help.