custody of our foster baby

We live in California, we have had our baby since he was 3 weeks old. He was born addicted to heroin, was on methadone until he was 6 months old. He now appears to be in excellent health, has bonded with our family, our 4 teenagers and adult children. His case has been drug out and he most likely won't go home until June. Our question is do we have any legal rights? The only Mom and Dad, sisters and brothers he knows are us. He sees his bio parents approximatley 2 hours a week. They have been drug free over a year, the father has been working about 6 motnhs or so. They live in a very poor, dangerous area, even though they talk continously of improving their housing. His mother has been an addict for years, has lost two daughters to adoption, recently lost a baby at 5 1/2 months pregnancy, drug related. When he was origianlly was placed with us we were told he was going up for adoption shortly, for the above stated reasons. We believe we can provide the best home, our unconditional love and long term things., ie. education, health care, consistency, and stability. We don't hate his parents, we just question their ability to parent. Can we fight his return to his biological parents???