I honestly don't know if you can easily transliterate his surname from Polish to English. Mostly because of the letter "ę" which, if I had to, I would say sounds a bit similar to the sound "ew" when you think about something disgusting, but still it's not quite the same.
As for the SZCZ part "shch" is about right. The NY ending is more like the WY in the word WYRM (fantasy fans should get it). So the Y is really short and hard unlike "knee" where the sound is longer and soft.
So to summarize it would be something like SHCHEWZNY.
Good luck.