Learn French: Son fils est un garçon intelligent.

It could mean either! In French, possessive adjectives like son match the gender of the noun they describe, not the owner. Because fils (son) is a masculine word, we must use son. We would still use son even if we were talking about "her son".

Why does fils end with an "s" if we are only talking about one son?

The word fils is an exception because its spelling always ends in an "s", even when singular. When spoken, the "l" is silent, so it sounds like "feess". If you wanted to say "his sons" in the plural, the word fils stays exactly the same, but the possessive changes to match: ses fils.

Why is the word order garçon intelligent instead of intelligent garçon?

In English, describing words (adjectives) usually come before the noun, but in French, most of them go right after the noun they describe. So you literally say "a boy intelligent".

What is the little mark under the "c" in garçon?

That little mark is called a cedilla. In French, a "c" placed before an "o" normally makes a hard "k" sound. Adding the cedilla changes it to a soft "s" sound so that it is pronounced "gar-son" instead of "gar-kon".