Little late to this but yes, blanket that horse!!
No sheet, he needs a blanket with warmth properties in it.
58 and no hair, no coat, and exposed to the night-air damp and nightly breeze most of Florida gets... He needs a blanket...
When the temperatures dip again like predicted to happen to south Florida he will need more warmth factor till his coat grows back in.
cooler the day of the show or take his sheet with you to keep him clean, more importantly comfortable in morning chill of air and damp factor again making a large difference in what the thermometer says and what it feels like.
By me this morning, 58 on the thermometer but 47 "feel like" is cold when exposed to it for hours.
Would you want to be outdoors for 10+ hours in that temp with no coat...think not.
Do your horses have stalls/barn or are they 24/7 paddock kept also makes a even bigger difference in what the horse needs..
Once the sun is up and burned through the morning mists it is warming, but till that hour of day arrives and temps climb, horse(s) need protection when clipped.