Yup. He should grow up first.
agreed.
Do not use socially mature. Socially mature means he has chances to mature socially and he doesn't. He is socially inept. Because he never had a chance to be adapt. You know, what with being bullied for being small and meek. Getting embarrassed by a girl who decided to make a joke out of him because of a punishment game. Being bullied for things he likes. Being bullied in general: Seme saying : are you bullied AGAIN. Seme's friend bullies him just because he needs to take it out of someone.
But you're right. How awful that he's self-absorbed.
Soutarou isn't emotionally mature enough to be in a relationship at this point imo. Satoi's assessment that he's self-absorbed is right. I know quite a few people like that irl and it never ends well.