Oh, dear. Poor, poor Winston.
Lady has been epileptic for nine years and what you are describing doesn't sound like an epileptic seizure. The seizure itself can lower blood sugar, but you said his BG is low before he seizes.
I would certainly not advise prednisone long term as it can cause other problems like diabetes and Cushings disease.
What do you feed him now? Lady is is also a Type I diabetic and gets insulin twice a day. Diabetics need a higher fiber diet to keep insulin levels stable. Since you have certainly been through all the tests and consulted a specialist, I think I would almost try to see if you can control his seizures with a change of diet. My first preference would be homecooked, but if that's not doable, there are a couple of commercial diets available that are high in fiber and lowfat that work well for diabetic dogs. It certainly would be worth trying to see if a high fiber diet could prevent his crashes.
Science DietŪ Results Nature's Recipe Senior Lamb Meal & Rice Recipe - Food Center - Dog - PetSmart