Frilly -you need to get legal advice fast if you want to keep this child .
Have you contacted your professional organisation ? They should be able to advise you on the steps for a non agency adoption. It's a non agency adoption because the child wasn't placed with you for adoption and you don't have the LA permission to adopt this child.
AFAIK you can petition the courts NOW to adopt the child, as he has been living with you for two years. The court will require the LA to submit a report to them within a certain timeframe. Once you have submitted a petition, the child can't be moved without the courts consent .
AFAIK,you can't do anything once he has been moved . If the LA think you might do this, they could move him immediately . I know FC this happened to, SS turned up without any notice and took the children immediately .
Just to warn you, once you do this , your fostering career is probably over. Your agency may never place with you again and they may give you a bad reference so no one else will take you on. If any of your children have SN and you have to go on dealing with them, this may make things very difficult . SS do NOT like to be crossed and they can be very vindictive and have long memories. I'm not trying to scare you, I just want you to understand the implications of being " uncooperative " .
Let me be clear -I am NOT a lawyer and I don't even know where you live, so my advice may not be accurate. So please get some specialist advice ASAP .