Welcome. Some of your son's symptoms sound like SPD, but not all of them. Have you ever done an SPD checklist? It may be worthwhile to do one to put your own mind at ease. You're right that it is not an approved diagnosis, but some of the symptoms of SPD are diagnoses in and of themselves (see my daughter's diagnosis below). Perhaps it would be a good idea to have him evaluated by a developmental pediatrician, who would be able to identify symptoms that may indicate a diagnosis other than SPD or in addition to SPD. Here's a link to the symptom checklist on this site: http://spdsupport.org/resources/symptoms.shtml
It isn't unusual for kids with SPD to have other diagnoses. My 5 y.o. daughter has an SPD diagnosis of dyspraxia. She also has ADD and speech apraxia. All these diagnoses are related to her neurological system. She was also tested for some genetic disorders than can produce similar symptoms as SPD, but they were negative. A developmental ped could determine if he's a candidate for genetic or other testing.
There are lots of things that could cause the chronic diarrhea. A friend I met online has a son with this problem, and he has fructose malabsorption. Others I know are gluten intolerant or dairy intolerant. The diarrhea problem should likely be evaluated by a gastroenterologist.
I know this can be overwhelming, but you will find a lot of support and encouragement here. Keep at it.