I am assuming that the conviction is in Georgia.If so, any attempt to have the charges reduced would be based Ga. law. From the facts you stated, I see nothing that would justify re-opening the case to have the charges reduced if it w3re based on SC law.The fact that the victim's mother wants the charges dropped is not sufficient to have the case reopened. This is something the state might have considered in offering a plea bargain prior to the convition.
As for the defendant having been a victim himself, this is evidence that might have been used in the sentencing portion of the proceedings as mitigating evidence in hopes of obtaining a lighter sentence. In theory, if you could show that this was evidence that the Defendant did noit have at the time of sentencing, and could not have discovered by excercuising due diligence, then you might be able to get a new sentencing hearing. This is sort of a moot point since he has done his time (he would probably have gottent the probation until 21 even with this evidence).
If the institution where he was incarcerated was in any way at fault or negligent in allowing the assualt against him, he might consider a lawsuit against the institution. In SC the statute of limitations expires one year after hi s18th birthday.