When to put a dog with seizures to sleep?

I have a three year old Border Collie/Rottweiler mix, Napoleon. He's had seizures since he was about 8 months of age and they've been pretty consistent. He'll have a good amount of seizures every month but the other night he'd woken up and began to have a seizure, this one kind of was the mother of them all. It lasted no less than 5 minutes but not quite 10 minutes and there was a very different look in his eyes, post seizure he'd just kind laid his head down and his eyes were still blank even though the seizure was over. He wouldn't look at me or anything he just lay there with this big look of despair and defeat. After that he was depressed until the next afternoon. He's otherwise happy and healthy, full of energy. So, my question is; when do you put a dog with seizures to sleep? I want to bring him to the vet and possibly get him on medicine but I'm afraid because the vet has told me that medicine will sometimes make seizures worse.