The obvious overlooked question is 'How big is the 11 yo?'
If he is small(ish) a youth model gun would be in order. If he is anywhere near adult heights (5'6" or above) a more appropriate size would be in order.
The next question will be recoil tolerance. I have watched men substantially larger than me ( I stand 5'4") wimp out under the recoil of my Mossy 835 loaded with 3" shells; they wouldn't even try the 3 1/2".
Yes a 12 gauge semi auto with reduced recoil shells will have less free recoil than a pump 20 gauge with 3" shells, but it will also more likely than not have cycling problems too. Will the gun only be used for turkey or will it also be used for grouse / pheasant / dove/ what ever else? If other uses, you must look for the overall balance of size and barrel length appropriate to the multiple tasks he will use it for.
I can use my 835 for quail but it is not really a good choice. OTOH, turkey, duck, or goose it fits the bill just fine.