Every one of them was the right move
You took the phone away. It bought you a quiet evening, and that was all it bought, because everything he'd watched that week was still in his head.
You set a limit. Two hours a day back — and no way to know which two hours mattered, so you cut the good ones and the bad ones together.
You asked what he watches and got "stuff."
You read the book, and it was right. It was also about a generation, and you came away certain something was happening and no closer to knowing what was happening to him.
You thought about a therapist — and if he's in trouble that's still the right call. What stopped you is that this isn't trouble yet, and you can't book an appointment about weather. Her first question would be what he's been consuming, and you can't answer it, which is where the first three sessions go.
Every one of those was the right move with the information available. That's the difficulty.
Each of them acts on the boy. Not one of them can see what is acting on the boy.