Assuming that this gelding has had a chance to grow a winter coat where he lives, it should be no problem to haul him in a stock trailer in this temperature range. The lower critical temperature (LCT) for a mature horse is estimated to be between -11 and 0 degrees Centigrade (12 and 32 degrees Fahrenheit), so the temperature range you listed isn't even out of his thermoneutral zone. The LCT is the lowest temperature in the thermoneutral zone for an animal and is the temperature below which metabolic heat production must be increased to maintain body temperature. The air movement will make the temperature feel a bit cooler to the horse, but still his extra energy expenditure to maintain body temperature won't be very great. An insulated horse blanket used during transport can help to ensure that the horse stays within his comfortable thermoneutral zone also.
