legal rights to a step child

my son has been with this girl since she was 1 month pregnant with my grandson and he is now 9. she gave temperary custody in 2010 to a couple that she did not know when she left my son and would tell anyone where he was, we finally found out where he was when her and my son got back together and she decided she wanted me to take care of him instead of this lady, this was 1 year later , i printed out some paperwork from the internet that said she was signing him over to me permantly until the age of 18, she agreed and we signed and had notorized. we had him from february 2011 til August 2012 when I decided to let her and my son be parents so he went to live with them. The following Jan she left my son for another man and left her son with my son to take care of him she says he is going to live there with him for a year or so or until she has a place of her own but she is playing mind games threatening to take my grandson form my son. Does my son have any rights to this boy at all?