what do you say to an 8yr old?

My husband and I have 4 beautiful young adults of our own. when our youngest daughter turned 16 we met a young lady who at the time was not ready to have a child. she was a close friend of the family. as time went on we noticed how much of a mom she was NOT she herself also noticed this. My husband and I along with our kids decided to take on the resposibly as a family to care for this little boy. He has been with us for 6 years now and we consider him our son, we have not completed our adoption. When we lived in Oklahoma we began the adoption process twice. Each time she came back and took him from us. Voiding out the adoption process. After the last time she did this and we got him back, we moved to Texas. I am scared to complete the adoption because i am so afraid that she will wait untill the 6 month waiting period and come back to get him. He has begun to ask us questions as to who his real mom is. We are a hispanic family with a verity of looks, he is a beautiful blue eyed blond haird boy and he dosn't look like us. Friends at school always tell him that we are not his family because we are mexican and he is white. he had never questioned it before. Now he is asking so many questions about his real parents i dont know what to say to an 8 year old. We had decided that when he was old enought we would tell him everything...but at the age of 8 i dont know how much is to much and what he can understand. can anyone help. confused parents Rachel