Before his mother,Tom is always a boy.
Before his mother意为 在他妈妈面前,表地点,是地点状语
Tom 是主语
is 是系动词, 跟在系动词后的是表语 ,因此, a boy 是表语. 这里是系表结构做谓语.
always 表经常, 是一个频率副词, 做状语.