Sure he can: see Diagnosis & Prognosis in A&A p57-58. More importantly for Hermetic magi the sidebar says that Revealed Flaws of the Mortal Flesh "instantly diagnoses a disease (both cause and effect)."
I appreciate anything is storyguide discretion, but the wording is Bedridden. Not feeling a bit off, having a headache, only getting up to go to the toilet. The magi will need a bed pan. If it was modern day, there would be a bunch of beeping machines attached. Outside observers will be thinking is he going to die. The fact the magi miraculously pulls through when other would die, strongly suggest it is an aging crisis.
I just double checked. When the Longevity Potion is used up, it is a level of decrepitude. Not XP towards it, but enough XP to hit the next level. The guy is feeling the arthritis more, eyesight not so good, etc. The magi might not know the LP has gone by mystical means, however, it seems unusual to suggest the magi does not know it in their bones, especially as it's not just the time bedridden, the entire season is wasted.