12 ga. Just run a lighter 2 3/4" load and he'll be fine. Def find a SG that can run 3 1/2" loads as eventually, he'll want the flexibility of running magnums (geese, etc.). Get the best your wallet can swing. And I highly suggest the used market. The only new shotty I got was years ago, when a local sporting goods store was selling the Maverick 88 (bare bones Mossberg 500) with two barrels. An 18 or 20, and a 24"...for $189!!! I recently picked up a Girsan MC312. Not a bad budget semi auto, however, I sold to a friend after half a dozen hunts, as I found a Benelli SBE3, on consignment for half the price of new...and it had never been fired. The used market will get you something that you can use/keep forever. And higher end models are typically not abused.