Hello Fatfreek,
You say you are still on good terms with your ex-in laws, I would call them up and ask them if they are aware of this, I suspect they would know. If you get the feeling that they are aware of this child call your son up and confront him with this news, this is a grandchild that has been deprived the love of a grandparent.
Furthermore, your son needs a nice moral slap in the face for his holier then thou attitude for shunning you all these years while he may have a skeleton in his closet.