From what i could gather from this conversation , is that , your son is definitely not having developmental problem, as he has been above average student all along, by your own admission.
Neither, it is What we call is Conduct Disorder , halllmark of which is disobediance , undermining authority of elders, aggression and disrespect towards seniors and others, as , what traits you have mentioned about him are quite normal for a 15 year old boy, boys of his age usually behave in this way , as they are experiencing new things about themselves and it is a transition period from childhood to adulthood and one thing they hate most is when they are treated as kids by their parents.
On the other hand , parents misunderstand their rebellious attitude as something wrong in their behaviour and try too hard to correct them.
At this time you just need to give him , his space, start treating him as an adult, but not pardoning anything that is wrong.
As for the head banging trait , i would say that it is A Self Soothing act often developed by children , when still very young , and are similar to the acts of thumb sucking, obsession for a toy or a blanket, and is used by your son to get destressed and helps him to go to sleep and over the period of time , it has become a habit.
The second probable diagnosis can be an Attention Seeking behaviour.
Nonetheless , in both these conditions you are required to talk to your son in a calm and friendly way , talk reason with him, ask him to give up his habit, also give him some concessions and ofcourse the space a boy of his age would demand.
I hope everything will get alright.
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