He will needs to get a lawyer and then make arrangements to clear up the warrant. It is unclear when the warrant issued. But if, as you say, he did not know about the warrant, your son may have a basis to have the case dismissed for delay in the prosecution. Assuming your son did not take a deal before going to warrant status, he will still be able to have a trial and make the DA prove the case - they may not be able to do so given the passage of time. Depending on the circumstances for issuing the warrant, your son will likely need to post bail of some sort if case not resolved right away. If you son intentionally skipped out on a court appearance but now wants to resolve it, then he's likely to get more than he would have gotten had he resolved the matter in the first instance.
Your son definitely needs to clear the warrant and since this is a felony charge he needs a lawyer to assist him. The possible penalties for theft of property exceeding $400 not involving a firearm is one year in the county jail or state prison (which means 16 months 2 or 3 years). These penalties are maximums and depending on many variables not evident from your question your son could get the charges dismissed or get probation. He won't get either without a good lawyer; search the SoCal lawyers contributing to this site.