Haven't read any other response.
I'm an adopted parent as well.
My oldest son came to us when he was four months old. In those days you could still adopt through a county agency. The rules though were that you didn't get to know anything about the birth parents.
My son's foster mother did, however, grace us with a number of photos of him from the time of his birth to the time she had to give him up. Among those phtos were a few taken at his birth upon which was written the Birth Mother's last name. So...we figured out who his Birth Mother was,
We decided that rather than ever making it some momentous occassion when we announed he was adopted that we would try and make it a matter of fact.
When my wife became pregnant with our daughter, my son asked if he to had been in Mommy's belly. We explained, as matter of factly as we could, that no he wasn't but we got to pick him to be our child.
It's never been an issue (as far as we know) and my wife asked him a number of times since he was a teenager (he's now 32) if he wanted us to help him find his birth parents. His answer was always no.
Who knows how it has affected him. but if it has it's nothing which one could easily attribute to his birth status. So, I think we had the right idea. Whether or not it would work for everyone is far beyond me.
Your situation is a good bit different, but ultimately the same. Your son has to reconcile the notion of "real" parents with the only parents he has ever known.
This is, obviously, tough for a 13 year old kid who is all about his identity as a person (Thank your lucky stars that Mo isn't a girl though, because they, for whatever reason, have a much tougher times with this).
You can't (reasonably) prevent him from meeting his birth father, but, if you can, it makes all the sense in the world to meet with the guy before Mo does. Having said this, there is no assurance that that he will be on the same page as you. He gave up his kid. Sometimes this is a decision based on what is right for the baby, but often it is not. It wouldn't be unheard of for him to make some stupid move to make Mo his "buddy," and an authority figure who never tells him No
In the end the value of your love for Mo will rule the day. He's a teenager and all teenagers are crazy and so the next few years may be difficult, but if he was a product of your womb the same could be said.
I recall my son (around age 13) telling my wife that he couldn't tell her what to do because she wasn't his "real" mother. Her reply: "Well I'm the only mother you have and so you have to listen to me."
Kids will try whatever leverage they think they have to get what they want. It really is important that you don't allow any doubt or angst you may have to permit them success. You are his mother
and your husband is his father
Anyone who frequents this forum will recognize how much you love your son. He knows it too. Rely on it, and don't try too hard around this subject.
Don't shrink, for whatever reason, from the fact that you have been there as his mother for all of these years. Let him him see what he doesn't miss and you will be there when he comes back.
Don't, under any circumstances, try to compete with a "do what ever you want" birth-father who wasn't there when Mo needed him.
All the best.