I'm getting it from other names I've heard with similar spelling. Pretty much none of the letters have any phonetic cues (with respect to western phonetics). Having "zews" sound like "efs" isn't really that unusual IMO. Certainly no more than "b" sounding like "v" in Russian, or any number of other phonemes that make a bit of a jump from one language to another.
Try pronouncing "zews" fast, and I think you'll hear an "f" trying to push its way to the fore.
Seriously, I have no way of knowing how he actually pronounces it, since Polish names get mangled so much in America, but the actual Slavic pronunciation is Khwashchevsky (like the "shch" Krushchev).