As in almost all areas of life, it's better to do it right the first time. Undoing an injustice is always more difficult, but not impossible. California law says it's malpractice for an attorney not to file a motion to withdraw upon request of the client. Seek counsel in the local community where the case was filed. Best of luck!
If your son has been sentenced, it will be very hard to reopen the case. Remember you son admitted he committed the crime UNDER OATH. Remember also that you are not in charge of the case. I am a parent too and can imagine how frustrating it must be to see your son plead guilty to a crime. Your son and his attorney had privilged conversations in which your son decided it was in his interest to plead guilty. Your son's attorney can't tell you what he discussed with your son without violating your son's attorney client privilege. You son may not have told you everything they said. He shouldn't have becuase you could be forced to testify regarding it. Your son may have been too embarassed to tell you why he entered the plea. It is entirely proper for the attorney to listen to the DA in order to determine the strengths of the DA's case. Your testimony that it is "improbable" that your son committed a crime, may not in the experienced opinion of an attroeny create a reasonable douby that he was guilty.
However, some attorneys do improperly advice their clients to lie to a judge in order to get an advantageous plea deal. If you son belives that happened your son (NOT YOU) should consult with another attorney. That attorney may tell you that the burden has now shifted. Your son's attorney would have to show that his plea was improperly obtained and in most jurisdictions that he is in fact innocent of the crime to which he pled guilty.
THESE COMMENTS ARE NOT LEGAL ADVICE. They are offered for informational purposes only. Answering this question does not create an attorney-client relationship or otherwise require further consultation.
The only way to re-open a case once the verdict has been rendered and once the time for appeal has lapsed is through a Motion for appropriate Relief ("MAR"). To re-open a case the defendant must assert one of the reasons set out in the statute. One reason (NC Gen. Stat. 15A-1415(b)(3)) is that the defendant's conviction was obtained in violation of the US Constitution - ineffective assistance of counsel claim.