If his friends call him to go out,he will do it at once.
老师，不是非常了解代动词 do,它为何要加 it,可以省略吗；do 是及物动词，it=his friends call him to go out.是吗？go out=do, do it 为何不等于go out it.
与do有关的代动词有三种：do, do so, do it/this/that，这三者的使用方法略有不同。
第一，最基本的代动词是do，其基本需要是其代替的谓语不可以含有助动词：
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征，那就是它几乎极少出目前其它助动词之后，并且出目前助动词之后仅限于英国方言。比较：
常见承认：I havent read it but I will soon.
英国方言：I havent read it but I will do soon.
do so在方方面面像do，但它可以自由出目前助动词之后：
He will do so tomorrow. He may have done so.
有若干种状况，do和do so不可以相互替代：
do so后面不加宾语：I didnt invite Kim, but I did / ※did so Pat.
比较结构：She earns more than I do / ※do so.
表达状况：He liked it; at least he said he did / ※did so.
do不需要于非限定分句：There was no point in doing so / ※doing. To do so / ※do would be pointless. （英国方言可以用）
do后面通常不加状语：She agreed to help, but she did so / ※did reluctantly.
可以用do so的地方，通常都可以用do it，但更不是肯定的。
do it代替的谓语需要是一个主动的行为：He heard a noise and I did so / ※did it too.
有do it to sb.但没do so to sb.：I hope they dont do that / ※do so to me.
依据以上剖析可以看出，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，但不可以替换成do，由于它后面跟了状语。
