D.H.
I'm really close to you...I'm in Auburn!
You'll need to find out how to withdraw him...I've never done that, so I know nothing about that. Just call the Gwinnett County School system. As far as forms to fill out to homeschool, they are really easy. You can get them from your school system or get them online at a couple of different places. One is www.heir.org. You'll need a letter of intent and of course an attendance form. You should be able to get these forms from your local school system too.
American School, James Madision, Faith Academy, just to name a few for high school. Or you can now (new this year) do K12. It's like public school at home and I believe it's free. I don't know what route you are wanting to take, but the others up above that I mentioned will cost money/tuition. If you want to do it on your own, you'll be buying your own curriculum. There is a big curriculum fair on May 2nd and 3rd. I usually just go on the friday and get my materials for the year.
Your other option would be for him to get his GED and start junior/community college. Maybe he'd be excited to do that and feel like he is getting a head start with college before others are. College can also be done through correspondence if you want to go that route.