Have you considered that there might be more than one Jack HYMAN born the same year as your grandfather living in New Jersey?
At our last house (about 20 years ago) there were two Steven FREEDMANs living within a mile. And once I was on an airplane with someone else from Atlanta named Peggy FREEDMAN. We kept the flight attendants on their toes! I would keep this on the "maybe" list until finding more information.
I would also consider other spellings of HYMAN - maybe HEYMAN - when looking for both your uncle and the mystery man.
Peggy Mosinger Freedman