Reuven died leaving a secular will only The will was admitted to probate The halachik yoresh- a lone son – (along with other family members) signed a “waiver statement” which acknowledged that they have 6 months in which to challenge the will After 9 months the son Had a change of heart and decided to challenge the will May he still do so even though he signed the “waiver statement”?
In most batei din, the son will not be given the right to challenge.
This is because of a combination of the two factors. On one hand, the secular will is seen as binding by a number of leading poskim, and according to these authorities he cannot challenge it. See in partiucular Shul Cheshev Ha-Efod concerning this matter, who also cites from the Achiezer, and others.
Although there is sometimes an issue of who is muchzak, in this case it seems clear that the son is not muchzak, so that he will surely find it hard to challenge.
In addition, the signing of a waiver document will probably be considered a mechilah, although this can possibly be contested, depending on the circumstances.
This is of course not meant as a psak between parties, but by way of advice alone.