he then asked me how my brother was.为什么then 放在主语HE之后?

时间副词放在主语后面是可以的,比如
He,then,decided to accept the proposal.
于是,他决定接受这一建议.
His mother usually goes out on Friday nights.他母亲经常星期五晚上外出
这里 usually 这一时间副词就放在了主语的后面.