We know his can be used as pronoun and as determiner. But i am confused about how to decide whether it used as determiner or pronoun
he said that john was his father
Is "his" a pronoun or a determiner here?
Pronouns take the place of a noun.
So if you see his immediately followed by a noun, it's very likely a determiner, because two nouns in a row without a conjunction doesn't work.
Also if you can replace his with an article, such as a/an or the, and the sentence still makes sense, it's a determiner.
I think it is both. It is a pronoun because it replaces a noun phrase:
It fills the determiner/determinative slot in the syntax of the sentence: it cannot be used with an article like "the" or "a":
So "his" could also be said to belong to the word class or part of speech "determiner/determinative."
However, if you have to say one or the other, I would go with pronoun, since it is already a rule in English syntax that noun phrases (when put into the -'(s)-genitive construction) can play the determiner/determinative syntactic role.
(Some sources use "determiner" for the function and "determinative" for the word class, other sources use "determinative" for the function and "determiner" for the word class.)