Oh Muffy i am sorry. It is not your fault. This doesnt make you a bad parent whatsoever. Eating disorders are cruel and manipulative. It would have been surprizing if you had noticed straight away. You are doing so well. Stay strong for him and he will pull through. Its a struggle ahead but he can do it. Just know that if he is angry or struggles against gaining weight its the illness and not necessarily him speaking.
Take care xxx