Comment by bell-cot

> do I say ... ?

YES. Private, verbally. Intro with your long-ago medical experience in the area. Disclaimer with "obviously that experience is strongly biased toward illnesses, since they were in the hospital". Dunno whether you should mention anything as specific as "Likely he's digesting blood". Close with something about "not my business, but if you have not seen a doctor, then please do so, for your health's sake". Make it clear that he doesn't have to say anything to you in reply - not even a "thank you", let alone an answer or follow-up.