How can I get my son's custody back if his father eventually cleaned his name and his papers look good?

Children (boy 12/ girl 10) born in California. The custody case was originated in the same state. I moved to Mississippi in 2007 to escape from my children's father who was extremely abusive, he was convicted and I was awarded sole custody before I moved with my children, and we lived happily ever after with limited contact with the father until 2009 when he finally agreed to be "civil" and a parenting plan was agreed upon and accepted by the court, children visited dad during the summer. Many problems ensued and he took me to court about every 6 months(he would not have his half of the travel expenses usually and knew I would travel with the kids for court and of course let him have the kids while we were in CA). Fast forward Aug 2012, because he hadn't reoffended (abused me) in over 5 years (I was in Mississippi the whole time) the judge found that my "attempts to protect myself" we actually "parental alienation" and gave me two hours to turn the children over to the Father and that was that. My children now live in California and I still live in Mississippi. Now it’s been a year and my kids are miserable. My daughter expressed a desire to be here but she isn't very convincing in my opinion. I think being with her Dad has done her some good. However, my son is 12 and demands that I find a way to get him home. We have a great relationship, always have and he misses his "life" his friends and everything else a normal kid would miss when ripped out of his home. Dad looks great on paper so what can I possibly do?