It's easily assumed that Cousin Henry could possibly be Uncle Arthur's son. However, throughout the series, it's never revealed if Arthur was ever married... or if he ever had a son. Cousin Henry was played by Steve Franken, who was only six years younger than Paul Lynde. However Cousin Henry does admit Endora is his aunt. Endora also admits that Henry is her nephew.
Steve Franken had also played in several other episodes in Bewitched. They are: Samantha and the Loch Ness Monster (1971) (Bruce), Samantha's Shopping Spree (1969), (Cousin Henry), Darrin Gone! and Forgotten? (1968) (Juke), Samantha's Secret Saucer (1968) (Orvis), A Gazebo Never Forgets (1966) (Hawkins).
Steve Franken passed away in 2012.