If the younger son was a Jin of Kurama then this will reveal something else.
Didn't the manga stated that Uzamaki's were and still is the only ones to be a host to Kurama? I might be wrong.
It shares similarities with Madaras incomplete susanoo, and thats probably something kishi did on purpose... remember that they are the direct descendants of the sage!
his not a jinchuuriki..what was shown was his chakra transformation sort like his brothers susano'o besides sure it looks like kurama but look it also looks like a susano'o