If you found this cat as a stray or adopted it from a shelter that picked it up as a stray, it is not a Norwegian Forest cat. 3% of the entire cat population are a breed, purebred cats are not just roaming the streets and if you happen to find one that is it's probably somebody's escaped pet. Norwegians start at $1,200, that's a lot to spend on a cat just to lose it.
Kitten food also really isn't necessary. Kittens just need to eat more food, they don't need a special food. While vets are educated they aren't all experts in nutrition, they just aren't required to learn all that much about it since their focus is mainly on medicine. I would not take dietary advice from most vets personally.
Kibble is very unhealthy for cats, they've evolved to get virtually all of their water from their food, so a raw or canned food diet is best for them. I would stick to a quality grain free commercial raw or canned product unless you want to learn to make your own raw. This will be healthiest.