In Texas you HAVE to have the agreement up front and the "waiting until after the adotion" is a bunch of baloney!! Additionally, in Texas the child is eligible for state paid college tuition if an adoption subsidy is negotiated up front. I would highly recommend going to this website
http://www.nacac.org/subsidy_stateprofiles.html which lists the states and what the requirement is for eligible children.
Just think of this as your first battle in standing up for you son and taking control (negotiating) for what is in his best interest.