If I have physical placement of my child in the stae of WI what do I have to do to move him to GA legally?

I am assuming that the child's other parent is still living in Wisconsin.
You will need to file a petition with the court requesting permission to move the child. The court is primarily interested in the best interests of your child, so this is a very fact-intensive review. For example, if the child has a lot of family down in Georgia but not in Wisconsin, the court may find that weighs in favor of relocating. If you have a good job lined up in Georgia and can't find work in Wisconsin, the court will probaby consider that as favoring a relocation. The court will take all of the factors in consideration, along with any factors from your ex-spouse's perspective, in coming up with a determination.

You should read through http://www.wicourts.gov/formdisplay/FA-5008.pdf?formNumber=FA-5008&formType=Form&formatId=2&language=en and decide if you want an attorney's help with this. I'd recommend at least consulting with a family law attorney to help you get started.