My question involves a child custody case from the State of: Florida

My son is 11 years old and resides with his maternal grandparents, due to his mother being unfit. I live in Missouri and did not know that there was an open custody case for my child and once I did find out the courts felt more comfortable leaving him with his gandparents in Florida. I have always paid child support, but due to his mother being unstable and unwilling to let our son see me, I have not been part of his life like I should have been. His grandparents let him come stay the past 2 weeks in MO with me and my wife and other 2 children. He does not want to leave in the morning even though I know he has to go. He wants me to have custody and I don't know where to start since he is in another state. At one point MO was supposed to come do a home visit, but it never happened. I believe they just forgot about us. HELP please!