How do I get custody of my 13 yr old son who has been living with me for 2 months when I am moving in a week from Gwinnett to Alpharetta GA?

seek out a lawyer

File for paternity in your residing county if it hasn't been established

File for custody in family court in your residing county

If in a event if the custodian parent is unable to care for the child...you have to establish that reason or if you want partial or shared courtesy. The child should be able to attend school where you live, but the child's custodial parent has to sign and notarize a affidavit....even if you are moving to another county.

For starters you ask in the appropriate category, which would not be in Travel, but would be in Law & Ethics.

That's not a big move. Are you asking which county? Custody decisions are state governed, I think. I once talked to an assistant district attorney in downtown Atlanta about similar issues. I think that's where you start. It would help to have an attorney. There are issues when you move out of state, but not when you move to a near by town. 

Just kidnap him like everyone else does. Your right, the law was wrong

You need a family law attorney.