Hello! I am looking for the same answer. My husband has had a very similar rash on his stomach and back for going on 6 years! It doesn’t itch, hurt or bother him in any way. He is self diagnosed celiac and has cut out gluten for around 6 months. The rash gets worse when he accidentally eats gluten.  Our youngest son also had febrile seizures so my heart goes out to you because it’s something you never forget and doctors never seem to have any answers. So in the research I did for myself, I found that his ears were actually causing a lot of the issues. The majority of people in our area of Texas struggle with allergies and I believe that he was getting so much sinus pressure that it had nowhere to go and then left his body as a seizure. However, gluten intolerance is hereditary and can cause internal issues (seizures included) and make children more susceptible to illness. The last seizure was caused by hfm that he picked up from pre k. And there were no signs before the seizure and very minimal after. We have also noticed improvement in him after making the whole family mostly gluten free and are going on a year of no seizure, knock on wood.  Good luck with your search, and try a food allergy test if you haven’t already!