I think hes smiling beacause, well he knows exactly why Anthony is leaving-its not just to go Reno with his father-its beacsue Neri knows that Anthony's uncle is about to die. Neri looks at the way Fredo is acting and the smirk Neri is giving off shows him as probably thinking "what a pathedic smuck...". Neri doesn't smile too much but he has in the movie at appropriate times. Neri is just a stiff kind of guy who only laughs at sick things such as this.

...or maybe Richard Brite just had a giggle spell he was trying to repress, who knows...