How/When to tell my son that my husband is not his biological father??

I find this fascinating!On a recent trip back home where I haven't lived for 20 years, I briefly met up with an old ex-girlfriend. We've always been friends and stayed roughly in touch despite us moving to opposite sides of the planet.She revealed to me that her 21yr old daughter, was likely conceived in a brief "farewell" liaison we had before I moved overseas. She was married at the time - and neither of us wanted to upset that, which was a partial factor in my decision to try living elsewhere. I had always been told that young Ms E's dates didnt quite stack up and that I wasnt in the picture, but when sitting face to face again after all these years, I was told that the dates were a lot closer than previously indicated and over the years a number of characteristics have been observed that led to my ex believing that I was probably her father. This was a lot to take in!!! Ms E is fantastic young woman - intelligent, witty and ambitious. and in a really bold move we were introduced (just as an "old friend") - which was fascinating and a HUGE privilege.Obviously I dont want to upset any existing family dynamic and have no desire to inject the likely distress of deceit thats happened to both child and husband in this scenario, but there's a part of me that REALLY wants to know more!!!I wrote a really long multi-page letter to Ms E, which I have filed away password protected and will likely never send. I re-read it again for the first time since last summer (when I first learnt of this) recently and its fired up my curiosity again.Obviously this is a little different from an "adoption" reveal. What is the etiquette here?