No rule for this one
No ending or word group predicts the article of Sohn. Learn it as a pair and say it out loud: der Sohn.
der Sohn
It's der Sohn. Sohn is masculine, plural die Söhne.
Mein Sohn ist fünf Jahre alt.
My son is five years old.
No ending or word group predicts the article of Sohn. Learn it as a pair and say it out loud: der Sohn.