To get the most accurate answer to your question, I recommend you contact the school district to learn more about its enrollment policies, particularly with regard to children who have been previously home schooled. If you are referring to Cobb County in Georgia, here is the district's page in SchoolFinder (which includes the district's phone number for follow-up directly with them):
If you were referring to a different Cobb County, type your city, state and/or zip code into the SchoolFinder search bar to research the contact info for the correct school district. http://www.education.com/schoolfinder/
I'm also including some additional links below to support your family with the Back to School transition.
You need to determine how many credits your son has already received from homeschooling. Then you need to contact schools in Cobb County to determine how many credits and / or classes are needed to graduate. Every school is different as far as how many classes they have each day. Which also impacts the amount of credits. What I found in transferring public schools was that my daughter would have failed 4 classes, so I then chose the homeschooling route. By doing homeschooling, she actually gets to graduate earlier than if she had stayed in public school.