这是一个语言发展历史的问题，很久很久以前，正统的用法是使用he,his,当时大家并不因为是he就认为是专指男性，而是习惯上就看作中性的包括man and woman,但是随着女权运动一波又一波的兴起，尤其是美国的女权运动最为极端，她们要求和男性在一切范围内的平等，结果就是用臃肿笨拙的an awkward "his or her"来代替. 那时候也有their的说法，但往往被认为是错误的。列举一些语法家观点--寇姆认为单数是这些不定代词后的常规形式，而his or her 比his更普通。这些都是正式用法，他甚至都没论及their的用法。The singular is the regular form after the indefinite or general pronouns each one , everybody , everyone, anyone , either, nobody，neither, etc., since they are now usually felt as presenting the subject separately:‘Each of us must live his or her life.*‘Everyone has his hobby.* ‘ Either of the expressions is correct but the former is more common than the latter/ ‘Neither has a wife/...<<A GRAMMAR OF THE ENGLISH LANGUAGE>> Curme 1931

十几年后，<<A GRAMMAR OF PRESENT-DAY ENGLISH>>1947认为：此处使用their为错误，使用"his or her"显得不灵活，因此惯常的用法是使用he/him/his ，Everybody must make up his mind that he has many sacrifices to make if his country is to achieve final victory. [Not: Everybody must make up their mind that they have many sacrifices to make if their country is to achieve final victory. 

随着时间的推移，上世纪六十年代到八九十年代，上述观点在慢慢改变。像大家比较熟悉的牛津实用英语语法，朗文英语语法作了如下陈述：这几个表达具有单数意义，动词也用单数形式，所以逻辑上自然用he/she, him/her, his/her.，但在口语中复数形式更常见。<<A Practical English Grammar>>A. J. Thomson A. V. Martinet /1960-1986 。

1988年的朗文语法认为，该情况下，传统用法是使用阳性代词，除非上下文明确指阴性。但实际使用中，却代之以没有复数含义的指双性的复数代词they/them 。有些母语者不接受这种用法。

同时期和稍后些年出版的语法书则多倾向于用they/their/themself取代he/his/himself.兹举几例：夸克英语语法大全1985认为，“代词they通常用作第三人称单数代词，介于阳性和阴性之间的中性。这是一种方便的方法，避免了是否使用he or she形式的困境。它一度仅限于非正式用法，现在甚至在正式用法中也越来越被接受，与其在无标记意义上使用he或笨拙的he或she，许多人更喜欢在指代不定代词everyone, everybody, someone, someone, anyone, anyone, no one,nobody时尽可能使用复数形式，以寻求性别中立:[1] Everyone thinks they have the answer. 在正式的英语中，当性别不确定时，人们一直倾向于使用he作为未标记的代词。[1]的正式的等价语是: Everyone thinks he has the answer. 现在越来越被忽视。”很明显，they显示出已经逐步被接受，并可能替代正式用语。

九十年代以后的作者更多显示出接受复数形式，而排斥单数形式的迹象。After  everyone we normally use they/them/their, even though the verb is singular.

Before going into the temple, everyone has to take off their shoes. (We use 'their',referring back to 'everyone', when 'everyone' does not indicate a specific gender.<<Advanced Grammar in Use>>Martin Hewings  2002

在《Longman Student Grammar of Spoken andWritten English 》2003，中认为：they在会话中很常用，在严肃的写作中经常被认为 ‘不正确’ 而避免使用。但是-- However, there has been a growing adoption of this use of they in written texts.也越来越多地被接受。

2002《剑桥英语语法》更是双手赞成使用表单数意义的they，而否定单数的he，

在国内很是流行的<<English Grammar in Use>>  Raymond Murphy，最近四五两版也显示出解说变化：                                第四版-But we often use they/them/their after everybody/everyone:  Everybody said they enjoyed themselves. (= he or she enjoyed himself or herself)

第五版2019年-Butwe use they/them/their after everybody/everyone: Everybody said they enjoyed themselves. (= everybody enjoyed himself or herself)

注意，often消失了。

<<Doing English Grammar>>2021  同样确认使用they，English already has an answer to the problem: to use they to refer back to singular antecedents. This use has already drawn the notice of scientific grammarians; Huddleston and Pullum (2002: 493) give it their approval, as does Wales (1996: 125–133).

当然，也还有些语法学者早些时候持谨慎观点，如2012的<<Macmillan English Grammar In Context Intermediate.>>Traditionally he / his was used.  Someone / somebody has left his wallet on the desk.

This is considered 'sexist' by many people, and an impersonal they / their is often used instead. In formal writing he or she / his or her is used.

作为总结，还是看剑桥英语语法作者2022的最新观点吧：Usage Controversy Note

We’ve said that singular they is now widely accepted. But you will find it condemned in

numerous old-fashioned works on usage that are still in print, so it is worth pointing out

that the use of singular they with non-referential antecedents like quantifiers (everyone,

somebody, etc.) goes back centuries, and is used by writers of impeccable standing –

Chaucer, Shakespeare, Milton, Austen, and huge numbers of others. Even Oscar Wilde’s

impeccably pedantic character Lady Bracknell in ‘The Importance of Being Earnest’ uses

it when talking about ‘the end of the season when everyone has practically said whatever

they had to say’.

Defenders of using he as if it were gender-neutral are implicitly allowing an

extension in its meaning to cover neutral as well as masculine (social) gender, so it’s

somewhat inconsistent that they should condemn an analogous extension in sense for

they with regard to number, to cover singular as well as plural.

It’s also very surprising that they don’t seem to have noticed how ludicrous it

sounds to say something like

?? Your father or your mother might injure himself. And of

course switching from using he to using she solves nothing:

?? Your father or your mother

might injure herself sounds just as bizarre.

The truth, then, is that they is often neutral as to gender but he and she never are.

这个表示单数意义的they，被2019版的Merriam-Webster 收录为词义项

Only a few years before this would have been uncontroversially ungrammatical, but the

Merriam-Webster company recognized the new usage in its dictionary entries, and singular they

was chosen by Merriam-Webster as Word of the Year for 2019. The American Dialect Society

picked it as Word of the Decade for 2010–2019. Some people now specify in emails or social

media the pronoun they would like to have people use for referring to them. Using the wrong 3rd

person pronoun for a person is known as misgendering.

所以，说使用his现在肯定不能算错，是历史的遗留，但是很明显，越来越少。

用his或者their都正确。