What is the correct possessive pronoun or pronouns to modify guitar Neither Jack nor his brother will take guitar to the bonfire tonight?

The pronoun is his, a possessive adjective.

A possessive adjective is placed before a noun to describe that noun as belonging to someone or something.

The possessive adjectives are: my, your, his, her, its, our, their.

A possessive pronoun is a Word that takes the place of a noun that belongs to someone or something.

The possessive pronouns are: mine, yours, his, hers, its, ours, theirs.

Note: The clue to the correct pronoun is the singular noun 'guitar'. If the noun was the plural form 'Guitars', then the correct possessive adjective would be the plural form "will take their guitars..."