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

Interesting topic!I recently discovered I fathered a child 22 years ago. I have always been in touch with the mother (an old ex girlfriend) and we're great friends. The reasons for the deception are simple. The mother was married and there was no intention of ever rocking the boat.I would love young Ms E to know about me - and was even introduced briefly (as an old friend) last year. Its thrown me into a complete headspin.I'm powerless and have no right to demand honesty from anyone - but it does sadden me that a delightful, intelligent and engaging young woman who is my daughter - knows almost nothing about me. :(