Boxes 18 to 20 are the NYC local tax. Choose "NYC" and "New York" as the associated state.
Box 18 to 20 is not Connecticut-related. He earned his wages in CT, but the local tax is because he lives in NYC. Only Box 15 to 17 is Connecticut tax-related. If he lived in Connecticut, he wouldn't have anything in boxes 18 to 20, as Connecticut does not impose a local tax on its residents or employees.
