Susan, I'm glad you had a nice thanksgiving and that your son is feeling better.
I want to understand something you wrote in your original post - you wrote that he had an upper endoscopy in which the entire small bowel was visualized. I think this is highly unlikely. A typical upper endoscopy shows the esophagus, the stomach and the duodenum and that's it. That leaves about 20 feet of small bowel unseen by the DR. The small bowel follow through shows if there are areas of significant inflammation, but other than that you cannot detect small ulcerations or areas of relatively minor inflammation with this kind of imaging study (x-ray).
There is a new kind of upper endoscopy, done in just a few hospitals, where the small bowel is pulled up via a balloon device so the DR can see almost all of it. Since you didn't mention this (double balloon endoscopy) I doubt your son had it.
There is also a pill camera that adults can swallow to visualize the small bowel, but I would think a 9 year old could not do this test. Perhaps now that the small bowel series shows no strictures this is an option for you.
I understand why you might be worried about a Crohn's diagnosis vs. a UC diagnosis, but frankly I would suggest that each type of IBD is as bad or as good as the other. It's great that you got him on meds and that he's doing alright.