How do I tell my son about his complicated family story?

I came across this website accidently and am very glad I did. I adopted my son a little over 2 yrs ago. It was somewhat of an open adoption in that I know the bio-family. I am really looking for some advice from those that have been down this road before. My son had some contact with his bio-grandparents up until a few months ago. The grandparents are doing things that are confusing and harmful to my son so I cut off all contact until I figure out what is best for my son.

His Bio-mom has just recently come back into town and is trying to get her life together. She has 3 other children who are my son’s bio-siblings. As I know of, he has no memory of them.

I know it is important to somehow let him have contact with his bio-fam but I am not sure how. The siblings are 15, 13, and 11. His older brother is in a lot of trouble and doing drugs (the 13 yr old). How do I tell my son he has siblings?

Also, I am not sure how to allow the grandparents back into his life. And I am not sure if and  when to let the bio-mom ivolved. And if I do, do I tell my 4 yr old that she is his 1st mom? How close do I need to let the bio-mom be?

I am so confused and really only want to do what is best for my son.