How do you think it would actually change your life or your son's life to have your formal test results? In what way would it resolve impostor syndrome? I guess those are my questions. If it would make a difference to your life and view of yourself, then it may have some value, but I would remind you that what IQs are best at doing is predicting academic achievement in school. (Although clearly we use them for other things.) Now that you are past school (I assume), the utility of the IQ test would mainly be in understanding yourself--for which a single score may not be that helpful. If you have received mixed messages regarding your giftedness throughout your life, it may be that you have a 2e of your own, in which case the more valuable resolution of that would actually be a comprehensive evaluation now, rather than an IQ score from decades ago.
On a practical note, you may or may not be able to recover those records at this late date, depending on how long ago you graduated from high school. School districts are not required to keep cumulative and special services (including IEP and gifted records) files indefinitely. Depending on the state and the era, it is/has been usually somewhere between two and seven years post projected date of graduation. They are supposed to send you a notice that they are about to destroy your files, and give you an opportunity to come get them, or have them mailed to you, but they don't have to hunt you down, just send one notice to your last known address. (This is how I obtained my records.) And if you moved districts during your childhood, those records may or may not have followed you.
Even if the expected destroy date has passed, it may be worthwhile to call and ask for all existing records (you may do so as soon as you exit the system), just on the off chance that they are still lying about somewhere. Expect them to need some turnaround time to find them, though.
