I have sole legal and physical custody of my son. My ex had my 16 year old son sign an affidavit to request a change of custody without my knowledge. My son has stated he felt coerced and does not want to move. Is it legal for my son to sign such a document without my knowledge and what do we need to do to rescind the affidavit?
Your lawyer (and you need one) will know what to file in your ex-husband's court case. (Until he files his case, the affidavit is useless).
If you son does not want to move, once the petition to modify custody has been filed, he can file a counteraffidavit. At that point, it may become necessary for your son to testify at a hearing or at the minimum speak to a court appointed guardian ad litem.