Highly reccomend the Rossi but if he isn't a fan of kick get it in a 20 inch barrel and they make after market recoil pad because it doesn't come with one
Remington 700 CDL .270 Rossi M92 44mag CZ 452 deluxe .22LR Howa 1500 .204 Savage FVSR 22
You could always start him off on some 38specials then move up to 357 magnums once he's comfortable. My advice would be a model 92 over a 94 as they are smaller and easier to cycle. Not that there is anything wrong with a 94 or a marlin. Most 92s have a crescent but plate which can be a bit harsh. Try find one with a rubber pad or you can get a lace up pad. I've got a Browning 92 in 44 mag it's great to hunt with. 44 mag packs a big punch and leaves a big hole. 357 mag penetrate better with good bullets and are pretty good on pigs.
After rereading your post, mounting a scope later on a 92 is fairly hard. The only real options for mounting a scope are a 94 angle eject or a marlin with a flat top.
You can remove the rear sight and mount a piece of picatinny rail for mounting a scope, some low scope rings and a fixed 4x scope with no front bell works great
Remington 700 CDL .270 Rossi M92 44mag CZ 452 deluxe .22LR Howa 1500 .204 Savage FVSR 22
How old is your nephew? 10 years and recoil is a consideration but if he's 15+ then maybe he's ready for something more like a BLR.. If not a 30/30 sounds like a good option
I have a Rossi in .44 I use for cane pigs up north QLD. I'm a pretty small build, around 80kgs. It's an older one that has a metal buttplate and kicks fairly moderate, and the sharp edges from the plate don't help. I outlined the butt and picked up some off-cut leather with some thick leather cord and made up a butt cover with some foam installed. It's made my Rossi look a bit more unique to me. I can now shoot 60 rounds and walk away without a bruise. Although the cord is a bit thick I'll be replacing it with shoe laces at the back. Your nephew could get a .357 and make a good cover with a piece of foam installed to iron out any real recoil issues - just mind the change in Length of Pull
Marlin makes there guns in 357 or 44 and they have rubber recoil pads I don't no if you can get them there but I have seen them ported And I do no they sell a youth model with a shorter stock but I think just about any model will work you might want to take him to some stores and have him shoulder them and see how it fits.
I have been a fan of the Rossi .357 for about 21 + years ( Still using the same one ) And it will drop a pig as good as anything else, but if you want a scope ( this has been on my mind too ) How about the Marlin 336Y ( Youth model ) 5 shots, 12 1/2 inch LOP and a 16 1/2 inch barrel. I saw they are for sale at the site we are not allowed to talk about here for about $750, so pretty much the same as a Rossi.