Depending on the particular circumstances, there may be various options for trying to custody of your grandson (e.g., temporary/permanent guardianship, deprivation proceeding, grandparent custody proceeding), but it is best if you speak with a family law attorney to determine what option may be the best. Good luck.
You may have several options, but it is safe to say you probably won't get anywhere without a lawyer.
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As for the bruises on your grandson, you should report this to the law enforcement agency in the jurisdiction where he is in foster care. Don't expect DFCS to do anything about that situation. I have seen them cover that up before in a foster care case. You could also report suspected abuse in a foster home to the Office of the Child Advocate. They have a website: http://oca.ga.gov/ .