Yacht Ensign

Can a yacht registered in the US fly the Red Ensign?

Little something my dad was wondering. Ours is registered to Cold Spring Harbour, NY, and the owner (dad) is a US citizen, technically an expatriat, and not a British citizen. He would like to fly the red ensign from the stern, but the bloke at the yacht club in NY said he didn’t really know the protocol of the UK.

He can fky it as a “courtesy” flag from the yard arm, but not on the stern.

Lena fixing the yacht ensign