I read someone else's post explaining how as Ezio was dying, he basically hallucinated himself as a young man there, since in the end you get the credits “When I was a young man I had liberty but did not see it, I had love but did not feel it” and so on, plus how the man says "Courage, Ezio", before leaving, and would explain why Ezio confronted him with “I don't think it's the women that are the problem”.
Also for the theory he was poisoned, Ezio showed signs throughout the mini movie of being on deaths door from simple old age. Right before going to the plaza where he died, before his wife came into his office-like room, he showed signs of a heart attack and then again in the plaza. Ezio died of old age from a heart attack, a natural death.
I don't want credit for the discovery of who the man was so here's the site where I learned this: