Could someone tell me if his and their is a common noun?

Answer 1

"his" and "their" are possessive adjectives, as in "his book" or "their house."

Sra

Answer 2

"his" is the possessive form of the pronoun "he" -- it acts like an adjective

"their" is the possessive form of the pronoun "they" -- it acts like an adjective

They are not nouns!