Tom likes English. So do I.
Tom likes English. So he does.
这两个句子中的so不是替代词,而是副词,意思相当于 too 或者 also , 但是词序不同。即:
Tom likes English. So do I.
= Tom likes English. I like English,too.
= Tom likes English. I also like English.
Tom likes English. So he does.
=Tom likes English. He likes English, too.
=Tom likes English. He also likes English.
《韦氏高阶英汉双解词典》对 so 的这种用法有详细的介绍(第1986页右栏3:)
在英语语法书中有关“倒装”的章节中都有关于 so 的这种用法的内容。