If a man is graduating and wants to wear a prom dress while doing so, then let him ;-) Sizing might be difficult, but not impossible

Either they have modern ideas on dressing, or maybe it's a typo.
The OP has cruised several times, and could be very familiar with Nassau as a lot of cruiseships go there. And going dress shopping can be the outside the tourist-trap-box .
Back to the OP, according to google below stores sell prom/formal dresses.
https://www.facebook.com/Wedded-Bliss-116272185123640/https://www.classicitis.com/http://www.debonairebahamas.com/http://www.buttonsformalwear.com/