Brief Answer:
Observe the baby,head banging is normal in this age

Detailed Answer:
Hello

Thanks for asking on this platform

Head banging is quite a common complaint in this age.In most of the babies it is a part of behavior.
Babies who are under stimulated(hearing or visual defect,emotional deprivation,autism)do this for stimulation while those who are overstimulated(extra care by parents)find pleasure doing this movement.
Under stimulated condition like autism can be ruled out if his development is normal and interacts well with the parents.

I won't get overly worried as you have mentioned that he has been evaluated by a physician and everything(visual,auditory and neurological assessment)is normal.
Sometimes children do these kind of movement to seek the attention.
Most of the children grow out of it till 3 years of age.You can speed it up by pretending not to notice his head banging and at the same time observing if any thing abnormal is happening(lack of interaction and failure to achieve other social milestones).As the amount of time in this habitual activity get decreases,he will outgrow it more quickly.

I hope this helps.
Please let me know if you have any more query.
Wishing for your grandson's good health.
Regards

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