Just as you would if there were a medical emergency, you need to arrange an evaluation asap with a mental health professional. His primary care physician might be able to direct you to someone. If your family is insured, call the number on the back of the insurance card and you will be directed to a provider.
Stealing and lying could be signs of conduct disorder, oppositional defiant disorder, drug use...the key is to get him to a counselor or therapist. If he is physically violent or a danger to himself, you should probably take him to a mental health facility that is willing to do free evaluations. It is hard to determine without more info.