his vs theirs

his

pron
  • That which belongs to him; the possessive case of he, used without a following noun. 

det
  • Belonging to him. 

theirs

pron
  • That which belongs to them; the possessive case of they, used without a following noun. 

How often have the words his and theirs occurred in a corpus of books? (source: Google Ngram Viewer )