"his; her (masculine singular)" in French

“Son” is linked to the noun that follows, not to the gender of the person we’re talking about. e.g:

Marie asks Léa: Comment s’appelle le frère de Sophie ? (“What is the name of Sophie’s brother?”)
Léa says: Son frère s’appelle Pierre. (“Her brother is called Peter.”) because frère (brother) is a masculine word.

Marie asks Tom: Comment s’appelle la sœur de Christophe ? (“What is the name of Christophe’s sister?”)
Tom says: Sa sœur s’appelle Claire. (“His sister is called Claire.”) because sœur (sister) is a feminine word.