I have a Mossberg 500 20 ga. Bantam. Mine came with two sizes of stock-- regular and short, but I got it used, so I don't know if it came from the factory like that. It's pretty easy to shoot.
I started my boys out with a Rossi youth combo gun . It was a .410 and .17hmr combo set . I think they are around 200 bucks . We pretty much just used them to get them comfortable with a firearm other than a BB gun .
Once they started hunting , they used a Mossberg 500 bantam in a 20ga . We only used the 20ga for hunting . They did all their target practice with smaller caliber guns , I didn't want to get them gun shy in anyway . Whether it would be from recoil or the sound .
In a hunting situation I have yet to see my boys say anything about the recoil of a gun . My youngest is 9 and he has been hunting with a 20ga for 2 yrs. now .
The Rossi was a nice light easy to handle gun , but the .410 barrel was only good to about 15yds . It had a VERY open choke .
The Rossi we bought was a pretty small gun . I'm not sure what the LOP was , but it was short . Since your son is 12 , I would visit some LGS and handle a few to see what fits and what doesn't .
I was 5' 4" when I got my first shotgun.
Ithaca Model 51 20 gauge tricked by Simmons.
I ditched it and got an 870 magnum 12.
Yeah it didn't fit perfect, but clays and critters couldn't tell- I nailed 'em.
My ol lady is tiny, so got her an 870 Special Field 20 with killer wood, professionally cut down.
She hated the stock
Youngest wants to blast stuff, so I'm going cheap.........870 Express youth or an 1100 project.