Nouns ending in -on are usually masculine.
Words ending in -tion, -sion, -son, or -aison follow different rules.
Is garçon masculine or feminine?
legarçon
The rule behind it
91.9% of -on nouns are masculine
In context
On fait silence dans la bibliothèque, les garçons.
Be silent in the library, boys.
Chacun des trois garçons a gagné un prix.
Each of the three boys won a prize.
Chacun des trois garçons a reçu un prix.
Each of the three boys got a prize.