The guy may very well be a Kennedy...BUT...proving it is JFK instead of any of his brothers is the Jefferson dilemma.
You’re right about proving he is actually JFK’s child. However, I just looked and saw that Patrick was born Nov. 25, 1960. I’ve read somewhere that Jackie was extremely hurt that Jack was not there for his birth. Do the math. That is roughly 9 months before this Jack was born. I didn’t do an exact count, but babies do not always arrive exactly when they’re supposed to, either. Do we know where Jack was when Jackie was giving birth to Patrick?
Obtaining a JFK hair sample wouldn’t be terribly difficult or expensive, e.g.:
http://historical.ha.com/common/view_item.php?Sale_No=629&Lot_No=25186&src=pr
Description
President John F. Kennedy: An Authentic Sample of his Hair. Three hairs from a lock saved by Kennedy’s longtime barber, Harry Gelbart. Impeccably documented, with a long letter from Gelbart detailing his relationship with JFK, a copy of his notarized statement, and a certificate of authenticity from University Archives, Westport, Connecticut.