First, King Faier can back out at any moment in the fight. And second, Prince Reifa isn't allowed to join the fight. On those two conditions alone, Yuuta had already been terribly disadvantaged before the fight even started. He was offered a handicap instead of help.
And yet, he chose to agree to those terms without even thinking.
King Faier, who was actually just testing Yuuta, tried hard to hide the happy expression on his face while staring at his son who was currently complaining his heart out. The reason why he gave Yuuta those conditions was because he wanted to see what it would take for Yuuta to back out of an unreasonable fight. That's why it surprised him to his core after hearing his unwavering response. There was utter confidence in Yuuta's voice when he agreed to King Faier's conditions as if he never had a doubt in his mind. It's as if he already had the victory within his grasp and that King Faier is just there to assist him on it.