Its just way to pay respect to someone in Japanese.
does this mean he respects boruto?
Kun (君【くん】) is generally used by people of senior status addressing or referring to those of junior status, by anyone addressing or it can be used when referring to men in general, male children or male teenagers, or among male friends. It can be used by males or females when addressing a male whom they are emotionally attached to or have known for a long time.
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he is trying to be close
or he is trolling since he is here to kill them
lol @ the comments
Maybe he wont even attack them, probably he will tell him about his fate and stuff, if Kara knows something about the seal? And since they said Momoshiki and Kinshiki interrupted their plans"