Wirelessly posted (BlackBerry8703e/4.1.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/104)
Greetings again, gr8aunt!
Port Authority Bus Terminal (PABT) is less than 1/2 mile from Penn Station. A taxi should only be about $5-6, tip included, depending on traffic and time of day. Unless your nephew has large or voluminous baggage, however, he is probably better off taking an uptown A, C, or E 8th Avenue subway one stop from 34th Street/Penn Station to 42nd Street/PABT. There is an entrance into the PABT from the subway.
Better yet, why doesn't he just wait in Penn Station for the next southbound Amtrak train? They tend to run pretty frequently.