No, it's not Reiji's fault. He's the one who was abused. Even though it's to save him, it's not his fault. It's clearly his dad's fault for abusing him, his mom's for choosing that solution, and Gen's dad for taking advantage of the situation. Reiji is not at fault.
We really need to stop this culture of blaming the children. They never asked to be born. If adults can't resolve their problems without involving the kids, they shouldn't make babies.