When they get into this type of situation, where they are having seizures and are in such pain, and you have no answers only more questions, this is when it gets tough.
The questions that only you and your vet(s) can answer are:
What is his quality of life like now and will it improve?
What will the seizures do to the brain and the body?
What if any are the medications doing to the body both short-term and long-term?
The problem becomes that once we make that very difficult decision of letting go and releasing, then it becomes our pain. This animal that has graced your life for so long, his pain has ended. But now YOU enter into the grief process and you are racked with questions- Did you do the right thing? What if you did something different? Why did this happen?
You may never know any answers here. When cats are left to breed indiscrimanantly out in the wild, this is the roulette wheel we take on when we adopt a kitten or cat. They may harbor something rare or unknown deep in their genes, and only after certain triggers go off, they become ill.
My vet, who has been through this type of situation with me so many times just labels cats like this as NDW (not doing well) There are fevers or unknown origins, seizure-type activities wracking the brain and the body, loss of bodily fluids, hours and days spent in prayer wondering what I should do next to try and "fix" this problem.
Some problems are not fixable, cats do not live forever. God, I wish they did. There are many diseases out there and illnesses that vet medicine hasn't even tapped into.
What I am trying to say is ultimately, this will come down to a family decision. Are your strong enough as a unit to release this cat from his torment and send him in peace, or are you waiting for a miracle to come along and cure him? He will give you the answers you seek, if you listen. Get down to his level, clear your mind and your heart and stroke him and look into this eyes. Are they clear, or cloudy with confusion and pain? Cats are low on the food-chain, it is instinct not to show pain. If they show pain, they understand that their time is limited. They remain strong to the end, some even purring as they take their last breath.
As I said, in my life's path I work with animals that have been horribly treated or abused. I have to make this decision more than most, I suppose. I just want to try and let you know that in your heart you know what to do. If it is The Decision, and I am NOT saying it is, then you have to find that inner strength and let him go. If not, then you play it a day at a time, and if that gets to overwhelming then an hour at a time. No one here can see your cat, but all here can see your love for Tigger
I wish you the best-