What Causes Numbness On The Left Side Of The Body?
I guess pharmacies in France don't have blood pressure machines for customer use, so he hasn't had his blood pressure measured. He is an athlete, though, and runs marathons, has never had anything like this before. No drooping in his smile, no facial effects at all, just below forearm.
Suspect multiple sclerosis
Detailed Answer:
I read your question carefully and I understand your concern, especially with him being far away.
Had it been only the forearm one might think of a nerve compression, but the fact that both limbs are involved makes a lesion involving the central nervous system more likely to be the case. The fact that it spares the face makes the location either in the medulla or upper cervical spine, or in the cerebral cortex sparing the face area.
However those technicalities are not that important to you I suppose, at least not as much as the type of lesion. Well brain imaging is necessary to sort that out, could be any type of lesion, but considering his young age the first thing which comes to mind would be Multiple sclerosis.
I remain at your disposal for further questions.