Nouns ending in -is are usually masculine.
Is avis masculine or feminine?
l'avis
Mostly used in the plural: les avis.
The rule behind it
94.1% of -is nouns are masculine
In context
Son avis sur le sujet est très pertinent.
His remarks on the subjects are much to the point.
Il se pourrait qu'il change d'avis.
He might change his mind.
Cause toujours, je ne changerai pas d'avis.
Say what you will; I won't change my mind.