Driving an enable pin low via arduino
Unless you require galvanic isolation (and it's highly unlikely that you do) you don't need the relay.

The EN pin only needs to be above 1.3V to turn the unit on, or below 0.9V to turn it off. The voltage range is anything up to 30V.

Just connect the EN pin directly to an Arduino IO pin. Write a HIGH for on and a LOW for off.  And of course connect the grounds together.