At 8 he needs to be shooting a .22 first. From there he could shoot a shotgun but things need to be considered.
For starters you need to know his eye dominance. He needs to shoot w proper eye not proper hand.
Once he’s shootings properly a 410 at stationary targets say a clay bird or can on a hill would be fine. Wing shooting a 410 is an expert gun.
Are you looking to let him hunt this year if your state allows it? If so he may or may not be ready. A 20 for an 8 year old may be too much. Take things slow and allow him time. Use inner and outer hearing protection to make gun seem to kick less.
Now shotguns need to fit. Lop should be correct for him. We also need to consider strength of the child. Some kids are weak. In that case most semis are heavier and thus out but they recoil less. Perhaps the child can physically carry a gun n can’t stand the recoil.
I know it seems I’m going back n forth but I teach all kinda kids and what you’re asking has many variables. Not one person knows your kid like you.
My recommendation might be a .22. Than possibly a .410 at a STATIONARY target.
When he starts wingshooting a .28 is really the most friendly gauge. It’s got enough pellets yet won’t pound him.
Take it slow w the child. Lots of kids are scared away from guns because of our desires not theirs. Grow em slow.
Myself w we shoot .22s. From there an AR15 than a bull barreled 22-250 to help em get used to noise.
From there a .28 w light reloads. Once there heavier loads are used eventually a 20.ga
Here’s another thing. Kids don’t need to watch a gun especially a turkey gun patterned. Sight in the gun. Give them LIGHT loads. When hunting give em the turkey load. The adrenaline overload will mask recoil.
Lots of kids are ruined from initial contact. Go easy and never test him