He converts right before you kill him in the Vanilla route. You however have to find the 5 "Secret Dasan Files" scattered across the game. He acrually ends up becoming a multi faceted character because of them.
Dasan file N 1 found in Sophie’s room:
General [REDACTED] decided to establish our base at the city of A.
We require troops and equipment for them, we will take them from the local smith, and we will pay him [REDACTED INFORMATIONS] after the war ends.
But the real reason why we are here is not to only to stop the enemy, but to search for an anomaly found near the woods of A.
If my concerns are correct, we may have to call archmage [REDACTED] to the cause, in the meantime prepare the Double Arsenal Specialist Anti Node.
Kind regards
~ [REDACTED]