The employee with a Tattoo

This is an actual event that occurred in Miami in 2017.  You are the Front Desk Manager at a 500-room chain hotel property.   Guest Service Representatives working at the Front Desk wear short sleeve uniform shirts and khakis.  While on vacation, one of your Guest Service Representatives, had a large tattoo placed on his forearm.  While visible tattoos are permitted, this tattoo is graphic in nature depicting an alien eating a scorpion.  You have received a couple of complaints from other employees as well as read two reviews on TripAdvisor mentioning the offensive nature of the tattoo in the hotel review.
You have asked the Guest Service Representative to purchase and wear a tattoo cover up sleeve when working.  He refused stating that other employees are not asked to cover their tattoos. 
Discussion Question: 
How would you address this issue with the employee?