7 year old son having fever related seizures

Hi, I'm new here, and this is my first post. I'm just wondering if anyone has dealt with something similar. My son will be 8 this month. Starting at a very young age he began having febrile seizures, and did until he was 3. They stopped at that point until last October. He had a very bad one in October when he was home sick from school. We called 911 and went to the hospital. 2 days later he was diagnosed with hand foot mouth disease. Which explains why he got so sick and spiked such a high temp. The doctor said it is not typical for a child his age to still have febrile seizures, and didn't think it would happen again. He did order an MRI and EEG as a precaution. Mainly because my son was born with transposition of the great arteries, and had open heart surgery at 7 days old. So he felt the cardiac team would request it. We have not yet gone for the tests, but should be soon. Earlier this evening my son had a temp of 37.7C. I gave him Tylenol and put him in a bath. As I was washing his hair he went limp and fell forward into the water. It seemed like he was starting to seize, but it stopped quickly. I will be calling his pediatrician in the morning. We didn't go to the hospital because he was fine within about 10 seconds, and back to himself after that. I have always found when he gets sick he very quickly spikes a fever to 38C, and once he hits that point seizes. But if I can catch it before it spikes and cool him in a lukewarm bath, as well as alternate Tylenol and Advil I can prevent the seizure. Maybe I'm reading too much into it, but it feels as though his body struggles with infection/viruses, and then he spikes, and seizes. Or could this be epilepsy, and fever/sickness triggers the seizure? Could it have anything to do with his heart? He has no issues. He has a mild leak I believe in his aortic valve, and mildly enlarged pulmonary artery. When he's much older he may need a valve replacement, but right now it's not a concern. But my brain keeps going to the fact that he always rapidly spikes to 104 when sick. It somehow feels important, but I'm far from a doctor. Hoping someone can make sense of my ramblings and offer some insights. Thanks 😊