Learn French: Après le match, tout son corps était fatigué. - After the match, his whole body was tired.

In French, the word tout (meaning "all" or "whole") is placed before the determiner, such as an article or a possessive adjective. You literally say "all his body" (tout son corps) instead of "his whole body". You see the same pattern in phrases like toute la journée (the whole day).

Why use the imperfect tense (était) here instead of the passé composé (a été)?

The imperfect tense is used for descriptions, background information, and ongoing states in the past. Here, était fatigué describes his ongoing physical condition after the match, rather than a single, completed action or a sudden change in state.

Does the word corps always have an "s" at the end, even in the singular?

Yes, un corps (a body) always ends in both a "p" and an "s", but both of those letters are completely silent. It is spelled exactly the same way in the plural: les corps (the bodies).

If the sentence were about a woman ("her whole body"), would the French translation change?

No, it would be exactly the same! French possessive adjectives agree with the noun that is owned, not the person who owns it. Because corps is a masculine noun, it always takes the masculine possessive son, whether it means "his body" or "her body".