It's too late to litigate the underlying case. He plead guilty and is on probation for it.
Being caught stealing is likely to result in a new case being filed against him as well as an allegation that he's in violation of his probation because of a new law violation.
Your son may have been placed on informal supervision without admitting the conduct in the old case. This means that it may not be too late to fight that old charge. In that case, your son will face charges in a new petition filed by the Distinct Attorney that will include charges relating to the old matter and charges relating to the new matter. On the other hand, if he was given informal probation after admitting the conduct in the old matter, he will now face disposition (sentencing) on the old case as well as charges relating to the new case. I know this sounds complicated, but without having access to the documents in his matter I cannot tell you specifically how this will play out.