Unfortunately, recoil and weight are a trade off and there's no free lunch as they say. If you get a gun that is light enough for a small framed kid to carry around all day comfortably, and it's going to kick harder. No way around it. You can use light target loads, but they're still going to kick pretty good from a very light shotgun. The felt recoil is a factor of the mass of the payload and powder, the velocity, and the mass of the gun. A lot of people buy a 20 gauge to reduce recoil, but they can actually wind up with a harder kicking gun depending on what shells they shoot. A 20ga shooting a 1oz shot load at 1300 fps that weighs 6.5lbs will kick more than a 12ga shooting a similar load that weighs 7.5lbs. I'd say get him a nice, mid weight, pump gun like a Remington 870 if he's big and strong enough to operate it. Load it with the lightest target loads you can get to start and work him up to the heavier field loads. If he's too small for the 870, you can try an NEF single shot 20ga (my first shotgun when I was 8). It's simple to operate, but you're going to want to put a really good recoil pad on it because the stock "pad" doesn't do much. Have the stock length of pull cut down to fit his frame. A good fitting shotgun is not only essential to accurate wingshooting, but it will also help reduce felt recoil.

Mike