Can a signed letter which is noterized by both parties giving non custodial custody of a child legal and binding?

There is a 16 year old boy for which his mother has not been in his life and does not have any rights to her son and is on Drugs and alcohol. His father came to us and said he is unable to take care of his son financially, is unstable and cannot give Josh a stable home plus Josh wants to be with us and he is willing to write out a letter giving us "Temporary Custody of his son" and with him, myself and my husband go to a Notery have it signed with all the details to allow us to take care of his sons, educational, financial, medical and any other needs Josh would have. His father has also moved to another area of Florida 7 hours north of us. He has sole custody of his son. Is this "Letter" noterized, legal and binding in the state of Florida for us to be able to take care of Josh's needs