Oh Droile, this is obviously so hard. I do wish you were getting some proper suport to help you through this - like a Macmillan nurse, or someone who knows what they are on about. It's v cruel - this is the time you should be able to enjoy and share DS's experiences of things like Halloween and bonfire night, and enjoy the here and now of being his Mum, esp before he starts any of the more challenging treatment like the chemo.
Could you perhaps find a way to focus on the next milestone - the neurologists change of drugs, the chemo or whatever, and think 'well, we'll carry on as normal til then'? Set up the next milestone or intervention as the next big event on the horizon, visualise it as exactly that, a big screen on the horizon, saying 'visit to neurologist' or whatever...and a screen so huge that it blots out your vision BEYOND that milestone. Because in a way, that's what it is like, isn't it? You can't see what's next, or know, really, 100% what will happen in the future. So keep yourself in your current safe seizure-free world THIS side of the next milestone.
I'm not trying to make light of this Droile, I know no pat answer or platitude will do, but if there is one little way to give you one 'normal' feeling minute with ds, then it will be a valuable minute, for you both.