stepson abandoned

We live in louisiana and just got my 16 year old stepson for a visit who lives in alabama. he let it slip that he doesn't stay with his mom but with his girlfriend's family and has since november! according to his girlfriend's mom, he moved in there to continue school while his mom joined a religious group in chicago. his mom has had very little contact with him during this time and has not helped them financially despite the fact that we've been sending her child support every month. she is now located in louisiana, atleast for now, and my stepson is with us for his school break. we are desperately trying to find out what legal options/steps we need to take to get custody changed over to us. also if she could face any jail time for leaving him without our knowledge, taking almost $2000 in child support for her own use, and owing over $12000 in back child support for another child of hers. she has 4 kids but only still has custody of him. i dont want to give him back to her or these strangers but i dont know what we are allowed to do under our current custody agreement.