与do有关的代动词有三种：do, do so, do it/this/that，这三者的用法略有区别。

首先，最基本的代动词是do，其基本要求是其代替的谓语不能含有助动词：

I hated it and Pat did too.

※ He will take the exam and I do too.

这种代动词有一个特点，那就是它几乎很少出现在其它助动词之后，并且出现在助动词之后仅限于英国方言。比较：

普遍承认：I haven't read it but I will soon.

英国方言：I haven't read it but I will do soon.

do so在很多方面类似于do，但是它可以自由出现在助动词之后：

He will do so tomorrow. He may have done so.

有若干种情况，do和do so不能相互替代：

do so后面不加宾语：I didn't invite Kim, but I did / ※ did so Pat.

比较结构：She earns more than I do / ※ do so.

表达状态：He liked it; at least he said he did / ※ did so.

do不用于非限定分句：There was no point in doing so / ※ doing. To do so / ※ do would be pointless. （英国方言可以使用）

do后面一般不加状语：She agreed to help, but she did so / ※ did reluctantly.

可以用do so的地方，一般都可以用do it，但是也不是一定的。

do it代替的谓语必须是一个主动的行为：He heard a noise and I did so / ※ did it too.

有do it to sb.但没有do so to sb.：I hope they don't do that / ※ do so to me.

根据以上分析可以看出，If his friends call him to go out,he will do it at once这句话中的do it可以替换为do so，但是不能替换成do，因为它后面跟了状语。