Georgia law allows for punishment of incareration up to 20 years. To find out what will happen in your particular case you need to talk to the victim advocate in the office of the District Attorney where the charges are pending. I have attached the "find your prosecutor" link to this answer it can help you find the correct office to contact about this situaiton. Good luck to you.
O. C. G. A. 16-5-21. Aggravated assault
(a) A person commits the offense of aggravated assault when he or she assaults:
(1) With intent to murder, to rape, or to rob;
(2) With a deadly weapon or with any object, device, or instrument which, when used offensively against a person, is likely to or actually does result in serious bodily injury; or
(3) A person or persons without legal justification by discharging a firearm from within a motor vehicle toward a person or persons.
(b) Except as provided in subsections (c) through (k) of this Code section, a person convicted of the offense of aggravated assault shall be punished by imprisonment for not less than one nor more than 20 years.
It sounds like you want an acquittal. Use Avvo to find several defense attorneys in your county to interview, and hire the one you are most comfortable with.
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Actually, to me it sounds like you do NOT want them to go light on him. Be sure to communicate those thoughts to the prosecutor's office. If you want additional help in Cherokee or Cobb, feel free to contact my office.
You can contact me at my office at 770-744-4211 or by email through this website. The questioner and any reader do not have an attorney-client relationship formed by our communications on this website. Advice given by me on this website is general advice based on partial information. You should not rely on any advice given without first hiring a lawyer in the area where the case is pending, and providing that lawyer with full information.
It's possible to receive probation for a conviction for aggravated assault in Georgia. Based on the few facts you provided however it is probably not likely unless your stepdad can legitimately claim self-defense.