his

We don’t traditionally think of his as having synonyms, but there are several situations in which it can replace—or be replaced with—another term.

his

Sometimes you can reword what you’re saying to use his in a different way. For example, instead of saying This seat is his, you could say This is his seat. 

his own 

The phrase his own is sometimes used as another way of saying his. For example, instead of saying After tasks are assigned, James should focus only on his, you could say After tasks are assigned, James should focus only on his own.

theirs

Instead of saying things like This house is his and hers, we usually use the plural form theirs, as in This house is theirs. Sometimes, though, it makes more sense to separate who owns what by using separate possessive words. For example, instead of saying These vehicles are theirs, you could say This car is his and this truck is hers. 

They has a long history of being used to refer to an unspecified individual or to a person whose gender and other personal details are unknown or irrelevant, and theirs can also be used as a singular in this way, as in I’m not sure who that is, but I think this car is theirs. Sentences like this sometimes use constructions like his or hers, but the use of theirs has become very common in such instances, with many people preferring theirs as less awkward and more inclusive.

Strictly speaking, his doesn’t have a definitive antonym in the same sense that short is the antonym of long. Of course, possession and association are often distinguished by using contrasting possessives, such as in statements like This responsibility is his—it’s not mine. In this sense, all of the other possessive words could be considered the opposite of his, including my, mine, her, hers, their, theirs, our and ours.

On the other hand, one person’s possession of something does not mean that someone else can’t possess it as well. This is indicated in expressions like what’s mine is his.

The word his is a pronoun that can be used in two ways. It can be a possessive adjective, as in This is his car. It can also be used as a possessive pronoun functioning as a predicative adjective, a type of adjective that modifies or describes the subject of a sentence or clause and is linked to the subject by a linking verb, such as a form of the verb be, as in This car is his. This sense can also function as the subject, as in His is the red one. 

In contrast, the word hers can only be used as a possessive pronoun (as in That car is hers).

His is used along with third person pronouns (like he and him), which refer to anyone other than the person doing the addressing or the ones being addressed. Other third person possessive forms include their, her, and hers. In contrast, first person pronouns (like I, me, we, and us) refer to the speaker or writer themselves. First person possessive forms include my, mine, our, and ours. Second person pronouns (like you and yourself) are used to address the person being spoken to or reading. Possessive forms of these include your and yours.