Perhaps watch a few Samurai films by Akira Kurosawa with your son and discuss the violence that happens on film in the context it arises? It's also worth watch

Perhaps watch a few Samurai films by Akira Kurosawa with your son and discuss the violence that happens on film in the context it arises? It's also worth watching a couple of YouTube reviews of Ghost of Tsushima. I recommend the ACG and SkillUp reviews. I think if you do this you'll be better placed to judge if he's emotionally ready or not.