What causes sudden onset arm weakness?
Stroke is most likely
I read your question carefully and I understand your concern.
Considering that description of acute weakness, without any trauma, neck or shoulder pain I am afraid the most likely cause is a stroke. Probably not a major life-threatening one if his lower limb and hand movement remains preserved, but still that is the most probable diagnosis and every stroke requires urgent medical evaluations (should be taken to the ER), even if a small one, in order to observe for worsening during the first few days and identify possible causes and risk factors (like heart arrhythmia, high blood pressure, diabetes, high cholesterol etc) which should be addressed in order to prevent stroke recurrences. Imaging, cardiac and lab tests should be necessary.
Other possible causes could be peripheral nerve injury, like damage to the axillary nerve or radial nerve (though radial involves more than just the arm), but that usually is due to a trauma leading to shoulder dislocation, or at least a movement leading very aggressive stretching of the shoulder joint not out of the blue after waking up normally as you describe it.
I remain at your disposal for further questions.
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