Am I correct in assuming that a paediatrist is similar to an American pediatrician? A medical doctor that specializes in kids? If that is the case, you REALLY need to find a neuropsychologist or at the very least a child psychiatrist. A pediatrician is great for medical issues but there is no way they should be diagnosing much less treating mental health issues.
Please get the book The Explosive Child by Ross Greene and try using his methods. You might be thoroughly amazed at what you learn. For me it was an eye opener. My "assumptions" about the reasons for gfg1's behaviors were 100% INCORRECT and my assumptions caused many of the problems we experienced. Please keep an open mind that maybe, just maybe he's not being defiant intentionally. That assumption could do more harm than good. Been there done that and gfg1 suffered dearly.