Re: Any advice?
Looking back at baby photos together may be a way to ease into the conversation I think... My kids love to look at theirs. Well, my bio kids have pics of me pregnant and holding them as newborns and the first year, etc. my adopted kiddo, no pics if newborn with me, we got him as foster at 8 months.
I agree, it shouldn't be done as a "big deal secret announcement" thing, more a casual, oh, I met u when u were xx months, you were such a cute toddler, etc.. Your daddy & I got married after (cue wedding pics of the 3 of u together?)...
You are his mom, but yes, there is also a bio mom and it needs to be mentioned!