"En public, son attitude était si charmante, pourtant."
Translation:In public, his attitude was so charming though.
This is an odd translation. "Though" is very seldom put at the end of a sentence. A better sentence might be "In public, though, his attitude was so charming."
Because "attitude" starts with a vowel.
For completeness: A singular noun starting with a vowel or a mute H always uses the masculine form of the possession (mon, ton, son). This is to avoid the conflicting vowel sounds. Gender doesn't matter here.
sa eau→ son eau ma amie→ mon amie ta horloge→ ton horloge
But keep in mind that if an adjective (that starts with a consonant) goes between the possessive and the noun, the rules go back to normal.
mon petite amie→ ma petite amie ton grande horloge→ ta grande horloge