He can remove her as agent under his power of attorney by giving written notice to her terminating the POA. He should make a new POA appointing a trusted person to serve and take action to get the vehicles retitled in his name, pay his bills, collect his income, etc.. If she has taken any money (in addition to transferring title to the vehicles to her son) or only transferred the titles, or both, then she has violated her fiduciary duties as agent under the POA and can be charged both civilly and criminally, for her wrongdoing.