No rule for this one
No ending or word group predicts the article of Enkel. Learn it as a pair and say it out loud: der Enkel.
der Enkel
It's der Enkel. Enkel is masculine, plural die Enkel.
Der Enkel ist klein.
The grandson is small.
No ending or word group predicts the article of Enkel. Learn it as a pair and say it out loud: der Enkel.