Well, if the statute of limitations has passed on the check forgery, nothing can happen on that. He can be prosecuted separately on the credit card fraud and the theft, though - the fact that he gave most of the money back won't change the charges. It would still be grand theft.
Once the case is reported, it's not up to the victim to decide whether to prosecute. He can still be called to testify and, if he refuses, he can be found in contempt of court
and possibly jailed. So, that may or may not affect the eventual outcome.
The judge has a lot of discretion in devising a sentence
, and may or may not consider the age of the defendant at the time. His age makes it more likely that he might be able to get some sort of probation, if he's willing to plead guilty. But if you're looking for worst case scenario, the judge can sentence him to jail in a juvenile
facility until he turns 24. I just don't think that's terribly likely. For one thing, the juvenile detention facilities are so overcrowded that a judge probably wouldn't give someone five years for theft. But he could end up on probation for a few years. Here is some general information on California's juvenile justice