Thank you for your query.
Though you have given a good description of what is happening there is no mention if your son becomes uncomfortable or cries when he is unable to move his neck. If there is no discomfort then you can wait and observe otherwise it shall be better to consult a doctor who shall examine the movements of the child's neck in all planes/axis and order appropriate investigations, if required.
I can only think of one condition and that is torticollis
. However, in torticollis the child always keeps the head to one side. Torticollis usually responds to physical therapy
I hope I have answered your query to your satisfaction.
Wishing your son all the best.