Sorry, my post may be unclear. I'm very familiar with hypotonia as I have a child who has been diagnosed with it and we've been through quite a lot of testing with neurologists, geneticists, etc. In none of that have I ever heard that SPD causes hypotonia and I can't imagine how it would. It makes perfect sense that having hyptonia could interfere with sensory processing as part of the sensory information the brain receives comes from muscles. So, if a child's tone is very low their brain may not be getting as much information such as where the body is in space. To put it in simple terms, when the brain is getting less information from one part of the sensory system it can cause the other parts to go into overdrive to compensate.
Sensory processing problems are real, but much like hypotonia, it is at best a very fuzzy, nonspecific diagnosis. It is more a set of symptoms than an actual diagnosis with a clear cause. I don't think it is fair to say SPD could change a person's muscle tone.
If hyptonia is a significant enough problem that it is interfering with a child's ability to enjoy activities, I believe it deserves a complete work up because some of the causes of hyptonia are treatable. This is particular true for milder metabolic disorders which are more frequently being diagnosed. I'm glad we pursued testing because it has made a big difference here.
I hate to see hypotonia dismissed as not worthy investigation because hypotonic individuals can face serious problems as they mature. First, there is a greater risk of scoliosis (as you've experience). Also, as hyptonic individuals mature as adults they can also face greater wear and tear on joints much younger than people with typical muscle tone. Even without a causal explanation for the hyptonia, physical therapy focusing on increasing strength and using proper posture and ergonomics can make a huge long term difference.
If you have a copy of his bloodwork you may want to do some investigating online. It is not uncommon for families to be told by neuro that the numbers are in range, but to later hear from genetics that there wasn't adequate testing being run.
