Might be a bit of a spoiler:
From what I understood in the novel, even though he was under a spell to love Beatrice, in the end he still ended up falling for Kaela which played a part in him being harsher towards her as the spell is reacting to him disobeying the magic. He just doesn't show much of his affection as he thinks he doesn't have the right to even love her, though that changes later in the story.

Why does he like the FL already? Like if it was just regret and sorrow for now, it would make sense, but why was he jealous?