When should I tell my child?

My child was dropped off with me by a long time friend who I hadn't seen in years. He was 5 months old and she was on drugs and said she couldn't care for him. He has 4 bio siblings he does not know are his siblings. He has met 2 of them and spends alot of time with 1. He knows his bio great grandparents and calls them grandma and grandpa but we call them that as wrll so there have been no questions. My son is 7 almost 8. I am white with blonde hair and blue eyes and my husband is mexican with light skin but very dark hair. My son is blonde with blue eyes so he just thinks he looks like me. He has also met his bio mom on several occasions but simply refers to her as my friend that he doesn't like. I never talk bad about her although he knows she is in prison now. But he has seen how she lost his last baby sister to dcfs and says she is a bad parent. He has formed his own opinion of her. He met his bio dad once but didn't care for him. Bio dad has 17 kids and cares for none of them. I'm just wondering when or if I should tell my child. He has a relationship with bio family but just doesn't know they are his actual family. I have made it a point for them to have a relationship with him simply because it's not the family's fault that his mom is in a horrible place in her life and his great grandparents just adore him. Anyway, any advice would be greatly appreciated.