He acts as before. 没有错。

= He acts as he did before.

He acts the same as before. 也可以，但也不是更正确。

= He acts the same as he did before. the same为副词作方式状语，修饰acts。 as引导比较分句修饰副词 the same. 也可以将the same as看作一个整体，引导方式状语从句。

= He acts in the same way as he did before.

He acts as before. 是最简洁的表达方式。

附：《韦氏高级》例句：

• I don't like to dress the same as everyone else.

• The business is run much the same as it was 50 years ago.