I don't know either, if telling him you knew would be helpful to him, but I'm positive letting it out here is helpful to you. I hope you'll begin to forgive yourself; you were a child and did not have the coping skills to deal with it. I think you'll find that most parents here didn't have the coping skills, either. I know I didn't, when I discovered my then 20 year old daughter was shooting up.
I had to learn to forgive myself for all sorts of things I did, that enabled and contributed to her addiction. Now that I know better, I do better. You will be able to say the same thing, if you take the past and let it become a learning experience about yourself, let go of the guilt.
Are your parents involved in any recovery programs for themselves, like Alanon or Naranon? Is your brother? It might be helpful to you, to consider something like that, or utilize your school's counseling services.
Many hugs to you!