My lawyer told me that I don't have to disclose my criminal offenses because the court expunged them from the record. Is that correct?

No. The Commodity Futures Trading Commission requires that all criminal matters involving felonies and specified misdemeanors to which you have pleaded guilty, nolo contendere/no contest or been convicted or found guilty, must be disclosed, regardless of whether they have been expunged; set aside or sealed; there was a conditional discharge or post-conviction dismissal; a state certificate of relief from disabilities was issued; or a pardon was granted.  However, if you entered a nolo contendere plea to a misdemeanor, you do not have to disclose it if you were not convicted or found guilty.