I could see my Kais brindle coming through in his 5 week pictures. He is a Kurotora and has two white spots, one on his neck and one on his chest. This is a common trait.
Kais can change their brindle over the first few years, but from what I've seen in my two it's very subtle. My male is 28 months and has had some minor changes over the past six months, a few more blonde hairs on the face and extremities. His tail is also turning red. This has happened in the past two weeks.
My female's red stripes/undertones turned white in the past year. She will be 3 in November.
Kais are amazing dogs and are very rare outside of Japan. Are you sure your dog is half Kai? Was this an intentional mixing of two breeds? I hope you don't mind me asking, I'm a very nosey person.