Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
Without the ability to take a history and examine your son, I can not make a diagnosis or give specific recommendations but I will try to provide you with some useful information.
In
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Infants' gas relief, nighttime episodes can include behavioral episodes. What appears to be voluntary head banging at the time of going to bed could be behavioral; it could be a stereotypy, a repetitive movement a child may do when excited, tired, bored, or other states. It could also be the child's way to express discomfort of whatever cause, such pain that is not well expressed, daytime events that have disturbed the infant, and other such factors. Other episodes occurring at night in children include nightmares and night terrors. These often occur out of sleep rather than prevent sleep. In general, all these episodes, including behavioral, often improve/resolve with age.
The fact that your son is having day time tiredness suggests, as you mention, that the episodes are not allowing for adequate sleep. Options for further evaluation include you video-taping the episodes so that his/her physician can see the episodes and better understand them and try to address them, or evaluation at a sleep disorders clinic that has a pediatric sleep disorders specialist. Several interventions can be taken in this age group that are primarily environmental (something as simple as adjusting lighting in a room etc) or behavioral.
Continued follow up with your son's physician is recommended.
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.