He knows he's not the best one here, not by a long shot (pun intended), but he wanted to join anyway. Maybe to challenge himself, maybe to show off what he had learned in these months after joining the Inquisition... or maybe just because he could.

Using the bow Korrin gave him (although it had no fire rune) instead of the more sensible one he had gotten from Metaari, it was a clear part of showing off.

Kas doesn't do awfully on stationary targets, but he's not in the top ten since he misses the center a few times. Still, the boy seems proud of his accomplishments.