I know it is difficult, but try and remain calm, especially when he is having a fit and coming round from it.
This is full of useful info. including what to do when your dog has a fit.
http://www.gsdhelpline.com/neurological.htm
BUT don't frighten yourself too much. It could be something really simple that he will grow out of (I went through a period in my early teens of blacking out - terrified my parents, but we never found out what it was I simply grew out of it).
I am sure he is in good hands and it sounds like the vet and the breeder are doing everything possible to find out what the cause is.
I (and I am sure many others on here) know exactly how you feel - they are so dependent on us, especially puppies.
I think the most you can do is keep him feeling safe and secure - remember he will not know about the fit, he will just feel a little shaky and insecure when he comes round. Give him lots of love (I am sure you are doing!) and keep him calm and quiet.
Take care.