Do you really mean "call him something silly" or "name him something silly?" If the latter, I worry for you.

What you call a cat, as long as it's in the privacy of your own home, is up to you.
What you name a cat is of entirely more import.
I can do no better than to quote TS Eliot on this important point.

On the Naming of Cats
The Naming of Cats is a difficult matter,
It isn't one of your holiday games:
You may think at first that I'm as mad as a hatter
When I tell you a cat must have THREE DIFFERENT NAMES
First of all, there's the name that the family uses daily,
Such as Peter, Augustus, Alonzo or James,
Such as Victor or Jonathan, George or Bill Bailey --
All of them sensible everyday names.
There are fancier names if you think they sound sweeter,
Some for the gentlemen, some for the dames:
Such as Plato, Admetus, Electra, Demeter --
But all of them sensible everyday names.
But I tell you, a cat needs a name that's particular,
A name that's peculiar, and more dignifed,
Else how can he keep up his tail perpendicular,
Or spread out his wiskers, or cherish pride?
Of names of this kind, can I give you a quorum,
Such as Munkustrap, Quaxo, or Coricopat,
Such as Bombalurina, or else Jellyorum --
Names that never belong to more than one cat.
But above and beyond there's still one name left over,
And that is the name you will never guess:
The name that no human research can discover --
But THE CAT HIMSELF KNOWS, and will never confess.
When you notice a cat in profound meditaion,
The reason, I tell you, is always the same:
His mind is engaged in a rapt contemplation
Of the thought, of the thought, of the thought of his name:
His ineffable effable
Effanineffable
Deep and inscrutable singular name.