Dear friend, welcome to HCM.
has his cervical cord been checked? he has weakness in C5-6/ 7 nerve
supply area it seems. can he take back his arm from shoulder? or pull it toward chest from side? soes he have any other symptoms?
sometimes brachial plexus injury
(even compression in sleep , with arm folded under the head )can present like this.
pl. see a neurologist
to evaluate these possibilities.
rarely ortho causes could be present, like a spur etc.
but I think neuro and spine need a look , if it is 2 weeks and not recovering.
Take care and pl. keep me posted on your progress.