It was traumatic for my son, and the doctors never knew why he had one, no reason whatsoever. He didn't ate anything poisonous, his sugar was normal, and the only thing I remember about that day is that he was very, very upset. He was very fussy and cried a lot, then fell asleep and right after that he had a seizure.
Stress can certainly raise blood sugars.
In the case of your son though, I think that it is probable that his seizure was actually due to hypoglycemia , but, by the time his blood sugar was measured, the body's counterregulatory hormones had brought his glucose levels back to slightly above normal. You should talk to your son's diabetes team about how to prevent this happening again. Briefly, you need to do a number of blood sugars throughout the 24 hour periods, so that you can see if his blood sugar is running low at any particular time. This should include a few tests in the early morning, and you should try to see if any low blood sugars are related to taking an unexpected amount of exercise or eating less. In any case, your son's doctor also needs to decide whether there should be a change in the amount or type of insulin that is being given.