The situations are difficult, and the answer to these issues are entirely dependent on the details of the particular circumstances, including the personalities involved. I’ve know of several instances involving family breakups, adoptions, etc., and they have all been handled differently.
IMHO, based strictly on the age of the child, is best that the birth mother simply stay away. The child is way to young to be dealing with what is an adult situation (divorces, remarriage, adoption, etc.). You should have a heart-to-heart talk with the birth mother regarding this matter, and try to convince her of this.
I assume this issue could be forced if necessary, though that is the route of last resort. Since your current wife has adopted your son, I assume the birth mother has no legal rights to your son, though that’s an issue to take up with an attorney.
FWIW, I am an adoptive parent x2. When it comes to answering the kids questions, simply be up front about it. Just give the minimum amount of details appropriate to the age, and never speak ill about your ex (i.e., his birth mother).