Rachel,
I can relate! I had febrile seizures as a little kid then at the age of 25, out of nowhere, I had a generalized tonic/clonic seizure in my sleep. I went through the mill - CT scans, MRIs with and without contrast, three EEGs - nothing seen and no one could tell me what was wrong other than I have a seizure disorder. I was on many different anti-seizure meds for about 8 years when my doctor decided to wean me off because I found out I was pregnant! After being off meds for about a month, I had another seizure in my sleep.

I had a high-risk pregnancy with my daughter and had to take Trileptal during the pregnancy, but she's turned out fine

Trileptal is basically a form of Carbemazepine and I've been taking it with no problems at all. The doctors describe my epilepsy as "idiopathic," which means they don't know why I have it. The technology isn't good enough right now to be able to see a seizure focus. Your son must be in the same situation as me. There is lots of great info on the web, have you checked out the site for the Epilepsy Foundation?
http://www.efa.org They have a very active forum! I hope you get some more answers.
Jennifer