I am not sure how much Akita your dog has,
but when Akita's mature around 15 months or so,
they sometimes go through a change where
they are not so accepting of strange dogs
and are more protective..
I am a bit baffeled about your neighbors dog though.
I would think they would be friends forever unless they
got in a scrape. (akita's never forget and they can
get quite jelouse like any dog can)
My suggestions is to keep socializing/training.

Another possibiltiy is that he might not be feeling well.

Good luck and welcome.