I've been in your same shoes. My daughter at 5 was still not potty trained. We saw a behavioral psychologist who was convinced I was just overreacting when she had an accident, which was why she was continuing to have accidents. We finally discovered the Feingold Diet, and after keeping a diet diary for several days, I discovered she's intolerant of citric acid and it was causing bladder spasms. Since we've eliminated citric acid from her diet (no small task), the accidents have dramatically decreased, and when she does have one, it's usually related to a food with a small amount of naturally occuring citric acid.
I agree your son has many symptoms of SPD, most of which my daughter shares, and would benefit from an OT evaluation at least.
Here's a link to another post I replied to, which I'm including so I don't have to retype everything. Welcome!
http://spdsupport.org/forum/thread-834.html