I went through the same internal debate as you, and I spoke with my pediatrician, a psychologist who specializes in gifted children, an OT and read everything I could on both issues. Ultimately, the OT as well as my personal research served me best.
Besides the sensory modulation challenges that may present as OEs, there are many other components to SPD that can be physically tested so the answer may be more objective (e.g. tone, motor planning, gross & fine motor skills, finger position sense, balance).
Most importantly IMO, our OT evaluation also included an evaluation of regulation and emotional function observed during the OT challenges, because a key component of managing this issue is observing a child's current regulatory capacity and developing a plan to bridge those skills. Are you seeing any other issues such as anxiety and difficulty with transitions? Anxiety can indeed be its own separate disorder or a by-product of a child's reaction to sensory issues and lack of ability to regulate response to those challenges. During treatment, the OT models skills to manage level of arousal.
I'm approaching this matter as treating the underlying sensory and regulations issues first, and then see if any other issues remain. Each child's situation is different, so your approach may vary.
I'm not sure from your post in what particular areas your son is challenged, but you may have to see other specialists outside your OT to assess other concerns (ex: visual processing, auditory).
I don't have any experience with genetic testing requests in relation to SPD, but someone else may have that answer.
Another key issue is that some professionals believe SPD can be a stand-alone disorder and others believe that SPD isn't a disorder and is only co-morbid with something else such as autism or ADHD. Be careful with both the missed diagnosis as well as the misdiagnosis - everyone views the situation through the personal lens of their experience (including me), but there is significant overlap between all kinds of disorders as well as some aspects of giftedness.
There are quite a few books that contain information on SPD, but my personal favorites are the books by Lindsey Biel, Lucy Miller as well as the Eides,"The Mislabeled Child". The Eides and Biel also produced a seminar available on video, "Sensory Processing Master Class".
Best of luck in your search for answers!
