Belle, this is truly a frustrating situation. I know kids with SPD that do score off the charts in terms of IQ, however, I don't know your son or the severity of his SPD.
Five is a hard age sometimes to get accurate results. Perhaps you can have him test again in a year or so. Lots of extremely bright kids "underscore" at that age for a variety of reasons.
I would try to find a tester who specialized in gifted kids and has a good reputation. I would even travel if it's possible for you. It just might be worth it.
Hopefully his therapy will continue to help him with his SPD so that at some point in the near future, he will be able to better tolerate the testing situation.
Good luck.
