My son was recently involved in an accident with an uninsured driver.Can the uninsured driver sue his insurance co if he is at fault

Yes.  Being uninsured does not restrict or prohibit the other driver from filing a lawsuit against your son.  It only limits his right to recovery.  If your son does get sued because of this accident, he needs to send the lawsuit to his automobile insurance company that covered the car he was driving at the time.  The insurance company will then provide your son with a lawyer and a defense free of charge.