
I just realized its Raion because of lion...

I think it was something about tiger blaming/being angry with his father for the divorce. I think the father worked alot so his mother spent more time with the kid and was closer with him but then suddenly the father becomes primary parent/full custody. He is just being rebelious trying to disrespect his father. By the end tho i think he is more understanding towords his father. So he continues to call him by name maybe because of his rivalry over the teacher. So still a rebelious thing.

Raion-san's actual name is Raion which the kanji is either this 来音 or this 羅音 . Raion also means Lion in Katakana so his name has double meaning and kanji meaning usually lose its Japanese meaning because people focus too much on the English meaning Lion (Raion) So when Taiga refers to his dad Raion he says both Lion (probably sees him strong etc) but also calls him with his literal name instead of just saying 'dad'. Same also goes for Taiga whose name means Tiger in English but kanji loses its Japanese meaning because how it is perceived by society. These type of names has a special word called Kira Kira Nemu which literally translates to sparkling name :) This is for 'bizarre' names. I haven't read the manga in raw so instead of using kanji to Raion and Taiga's names the mangaka might have only used Katakana, I really don't know. If she/he used katakana then it is 100% kira kira nemu without any Japanese meaning. Hope this is clear
I somehow like the fact that he asked him whether he wanted to top or bottom