Thanks for the kind remarks

Thanks a lot, David. Great contribution on this board, by the way! Keep it up

Your explanation is fine by me. Any guess by the viewer involves mindreading.
I like to think that he spent decades living in the same place, patronized by Tony, and considered an incoherent customer by the grocer. He is about as socially immature as it is possible to be.
He knows he has to get rid of her, is probably as nervous as he was capable of getting in managing this unusual situation, and is trying to feel his way to a plan she will accept. Her playful comment upset his delicate psyche beyond his ability to cope.

Yeah, I understand that.
But, I mean, why does Mathilda saying "Cute name" is THAT unexpected from him. Does he realize at this moment that she could be flirting with him? The "unexpected" part coming from the fact that she's so young.
I'm not a native anglophone so some subtleties might escape me.
Thanks.

A time-honored comic reaction to something unexpected. Danny Thomas perfected the spit-take in his early TV sitcom sixty years ago. Part of the fun in this film is Leon trying to recover his well-ordered and peaceful hermit life and Mathilda driving him to distraction
Look up spit take in Wikipedia.