What can I do about a child that was given up for adoption 6 months ago that my son is the possible father of?

I would seek help from an attorney in your area as soon as you possibly can.  Reversing an adoption can be a very difficult and costly.  There is also a time limitation in doing so (called a statute of limitations) but that may not have started until he became aware of the child and adoption.  There has to proof of parentage as well.  Good luck.