It seems to me as , your nephew is suffering from A Conduct Disorder, which is a psychiatric condition seen in adolescents(youngsters/teenagers) .
Conduct disorder is a psychiatric condition characterised by a pattern of repetitive behavior where the rights of others or the current social norms are violated. Symptoms include
1)verbal and physical aggression,
2)cruel behavior toward people and pets,
4)lying, truancy, vandalism, and stealing.
Conduct disorder is a major public health problem because youth with conduct disorder not only inflict serious physical and psychological harm on others, but they are at greatly increased risk for incarceration, injury, depression, substance abuse, and death by homicide and suicide themselves.
After the age of 18, a conduct disorder may develop into antisocial personality disorder.
I believe your nephew is also a victim of such a disorder.
So. my earnest suggestion to you is that you show him to a Psychiatrist or Clinical Psychologist, who will assess him by carrying out a thorough physical and mental examination and required investigations.
As for the treatment of such a condition is concerned it mainly comprises of Psychotherapy( Professional Counselling) which is supposed to be carried out by a trained psychiatry expert.
For your knowledge the different types of psychotherapy that may be given as a remedy to treat conduct disorder are-
2)Parental therapy and training programmes
5)Residential care- such as residential placement in a foster home, group home or special school.
This condition needs utmost attention of your nephew's parents as at this age he needs to get his act right before it is too late.
I hope you consider the options which i have mentioned in my answer and hopefully are satisfied with it.
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