Old English his (genitive of he), from Proto-Germanic *hisa (source also of Gothic is, Old Saxon is, German es). Originally also the neuter possessive pronoun, but in English it was replaced in that sense c. 1600 by its. In Middle English, hisis was tried for the absolute pronoun (compare her/hers), but it failed to stick. For dialectal his'n, see her.
In 16c.-17c. commonly used in place of a genitive inflection after nouns whose nominative ends in -s (for example, "When this Book became a particular book, that is, when Moses his book was divided into five parts, I cannot trace." [Donne, "Essayes in Divinity," "Exodus," 1651]). Here it is perhaps an expanded vocalized form of 's, originally -es. This tendency began in late Old English and was obsolete from c. 1750.