I want to know if my 9 year old son has behavioural disorder, which specialist do i have to consult?

Start with GP: Start with your GP. They may be able to do some screening tests and determine if your son needs to go to a psychologist or psychiatrist or a developemental Pediatrician. They can determine the specific disorder or disorders, and determine a course of treatment that my include behaivioral therapies, counseling and possibly medications if needed.

Mental health eval: A child psychiatrist or psychologist is a good choice. In the US there are developmental psychologists that often work through the school system.

Child behaviour : I would first consult with your primary care physician. If there is an obvious diagnosis, your PCP might be able to manage it. If not, the nest step would be a child psychiatrist or psychologist. Your PCP likely knows some specialists in these areas. It's also important to talk to his school to see what observations they have made concerning his behaviour.