My son turns 12 in January.  He needs a shotgun, his first hunting season will be spring turkey.  I need to find him a shotgun for Christmas.

He does a lot of shooting with me.  He has shot my 12 ga. 870 Super Magnum with field loads, my Saiga 12 with field loads, and my Mossberg 500 20ga. with magnum loads.  Of course the stock on the 870 is big for him, but he makes it work.  The Mossberg 500 has a youth stock and a regular stock; it was my first shotgun, bought for me by my dad.  It has a modified barrel, no vent rib and no choke tubes.  Poor finish on the barrel as well.

What I think are some good options:

1.  Buy him an 870 12 ga. Super Express Magnum, put a youth stock on it and give him light loads, then allow him to work up as he grows.  Eventually he'll be able to shoot 3½" shells out of it.  
Pros:  He will be able to use the gun as long as he hunts, he won't have to buy a new one later unless he wants to; light loads will make it usable for him now.  
Cons:  I can't find a youth stock for this gun.

2.  Buy him a youth 20 ga. model of his own.  
Pros:  Prepackaged to his size.  He would have one to pass down to his kid(s) someday.  
Cons:  He'll want a 12 ga. eventually and this one will end up sold or sitting at home.  Yeah, I use my 20 ga. from time to time on small game, but we don't hunt small game often and mostly turkey hunt.

3.  Buy a new vent-rib barrel with chokes for my Mossberg 20ga, allow him to use that as long as he wants.  Buy him the Super Mag for Christmas so he has his own to grow into eventually.  I have a two year old son too, so he could get use out of the rebarreled 500 in the same manner since I'll need to do this for him too.  
Pros:  His first shotgun I buy for him will be one he can use as his main shotgun for the rest of his life.  
Cons:  The first shotgun he hunts with won't belong to him, it would be mine since his younger brother will need it for the same reasons; also it would suck to get a gun for Christmas that you couldn't shoot comfortably because the stock is too big and you have to hunt with dad's gun and leave yours at home for a while.

I think the practical thing to do would be to make a re-stocked 870 Super work for him... but I think the right thing to do is just get a new 20 ga. Youth model and give it to him. (I probably need to buy the vent rib choke tubed barrel for my 500 anyways, but that's another matter).

What do you guys think?

ETA:  He has shown an interest in double-barrels.  Are there any double-barrels out there with choke-tube barrels and stocks that can be adjusted for growth?

Quoted: I like 2 or 3. As you have already found out Remington does not make a youth stock for the 12 gauge 870. But guess who does? Yes, Mossberg. Mossberg does make a 500 in 12 gauge Bantam and it does come with the 50% off card for when you replace the youth stock. MIKE. http://mossberg.com/bantam_models.htm

That looks to be a pretty good choice.  I wish they made it in camo/synthetic or matte black, since we're turkey hunters.  That would be a slam dunk.

I bought my son an 870 youth model in 20ga and found I actually prefer it to my 12ga.   It's a very handy package and there are heavy turkey loads for the 20 as well.   Nice gun and a great value in the express model.

Quoted: If you guys are turkey hunters than buy him the Mossberg Bantam turkey model. He will be happy as a pig in shit. It's only money. Shotguns are like golf clubs, you need several. MIKE.

That does look like a top choice.  What about the sights on it?  Won't those be tough for, say, wingshooting grouse or pheasant?  Any easy way to switch the sights?

If you can find one of the beretta A390 autos at walmart it might be what you are looking for.  I bought one for my wife last year and it comes with a butt extension for when he gets a little bigger.