[TacticalCoder]

Don't know how titles worked in the Austria-Hungary empire but his family had nobility titles:

https://en.wikipedia.org/wiki/Von_Neumann_family

(at least he's listed there)

Not sure which title and how it'd work in english but "Sir" doesn't sound off if he indeed was part of the nobility (not that you'd be forced to use it: but it may not be wrong to use it).

[klelatti]

From Wikipedia it sounds like the closest English equivalent would be Lord John Neumann of Margitta.

Reasoning: the title was hereditary and knighthoods aren’t but peerages can be. Also peerages are often associated with with a place but knighthoods aren’t.

[t-3]

No, he was foreign born, so any title would have had to be renounced to gain citizenship.