On one hand, you have every right to be called what you want, and people should respect that. On the other hand, I think you're handling this poorly. He only knows you as Holly, and as far as I can tell, Seiko is a nickname, not a full name change, and there's no identity issue (trans, shedding a name you hate). All you've told him is, "I'm Seiko now," with no context. To ignore him over it at this stage just strikes me as petty. You're not even giving him time to adapt, you're just punishing him.

Going with YTA, but a resolvable one. Either acknowledge to yourself that it's a nickname and let it go, or talk to him about how it's an identity issue (are you Japanese?) and why it's important to you that he use your new name.

And then consider just not giving him a hard time over it. I changed my name in my teens/20's (for personal reasons, not gender identity, etc.), and my parents still call me my birth name. I'm fine with it, because they support and love me, and I'd rather have that than to make it a hill to die on.