I too am so sorry about your health issues.
You described this man, to my eyes as a caring person. I am not sure how elderly he is, but do you think he would consent to a dna if it was done in secret from his wife/family?
He might not even be aware that you can just do a cheek swab and be done with it.
I am more of a calling person rather than a letter writer in our situation because I always wonder who else would see the letter, especially children of ill/elderly parents.
I hope you can find a way to solve this mystery...It just is not fair that in today's society that if a woman even suggests that a man could be the father, the court orders a paternity for money to be paid to the woman, and the father still does not have to provede medical, but we as adoptees have no rights in that regard for medical history and no money.
Best Wishes,
KM