I'd not accept that the puppy is 'too young'. Further if I had a PUPPY fitting, I'd suspect epilepsy - it's only when an old dog starts fitting, that this need not necessarily be epilepsy. I think you need to be referred to a neurological specialist rather than mess around with a regular vet. - much as some do have a specialist subject within a Practice.
Of course his breeder needs to know what's going on, in case this is epilepsy - if so, then that breeding should never be repeated. I ended my bloodline when I heard the outside male I'd used started fitting in later life and was put down. More worrying was the fact his mother did the same thing! Although we'd never had any sign of epilepsy in our bloodline, and it is around in the breed, I decided the buck stopped with mine. I refused any stud enquiries I had for the male, and had the b itch I'd also kept from that lovely litter, spayed without issue.
There are of course, other things than epilepsy that cause fitting, but the younger the dog who starts fitting is, the more worried I'd be about that being the cause. Did your puppy come from a reputable knowledgeable breeder - I hope so because with the best of intentions, when two lines that have never been combined before are, things like this could happen. I hope you can talk to whoever bred your pup. And you get reliable specialist help.
Add. Provided the seizures can be controlled, there's no reason why a dog can't live on with this BUT bearing in mind cost, you might like to consider asking the breeder to take the puppy back, if only to have the diagnosis done and let you know about the prognosis going forward. I realise you are now bonded with the pup, but look at the Sale Contract re guarantees. I'm sure a responsible breeder would want to be involved (I certainly would).