You need to see the psychologist who did the recent testing and discuss this drop in your son’s score. The difference may reflect a change in the way IQ is tested now, versus three years ago. For example, scoring for the WISC IQ test changed when the IV edition replaced the III edition, in 2003.
I am concerned if, after all options are considered, your son’s IQ has dropped 26 points. If this is the case, further neuro-psychological studies, plus a neurological evaluation, are needed.