While I was out of town for several months, a "friend" , against my specific instructions, drove my truck without authorization while he was not in possession of a license, for at least 10-12 weeks. During this time he did significant damage to the body of the truck, ruined the 4 wheel drive, broke the mirror off the driver's side and made a mess of the interior.
I said I would not press any charges if he returns my truck to the condition it was in when I left, but I am unsure if he will. If he doesn't who should I contact to have him held accountable for what he did?
In some state unauthorized use of a vehicle is a felony, but I am unsure of Georgia's laws.
Thanks for any help you can provide.

I don't see anything like "unauthorized use of a vehicle" in Georgia Code Title 16, "Crimes and Offenses", but you might want to ask you local police to be sure.

Sue him, probably in small claims court, for the damage to your vehicle.

Thanks for your advice.I will contact the local police and see what they advise.

I worked as a police officer for many years in Georgia.

The issue here, is you did not report the "theft" of your truck, the moment you returned, the delay in reporting, will cause doubt as if it was really theft.

Also, how did he get the keys to drive the truck, if you left the keys with him, that will also lower any case against him.

I do not see any DA in Georgia even touching this, They will assume, he used truck, damaged it, will not pay, and now you want revenge.
May not be, but that is what they and the courts would see.

You will have to sue him in civil court, for the damages. Your failure to report the damage and theft as soon as discovered seriously hurts any case.

Fr Chuck, as I read the question, OP left the vehicle with the friend, with instructions not to drive it. Thus "theft" is not an issue; it is a bailment.

Yes, I agree, going to be an issue with Georgia law, really civil, not criminal