The weather has been pretty weird on the south and eastern coasts this year and I know from when I lived in Louisiana that occasionally the humidity levels can shut down a horse's sweat mechanism....I am just guessing here so don't take that as a fact. A few things you can do to help: wet down his chest and flanks before you ride; increase, or add, an electrolyte to the feed..this may help increase sweating but will also increase water intake..neither of which are a bad thing :). It may also sound strange and I would check with your vet but we had a horse come down from New England to Louisiana in the high heat of summer and his sweat mechanism completely turned off. The vet "prescribed" a 12 oz can of beer a day added to his feed...it worked; again, do NOT do this unless under the care of a vet. He wasn't the best sweater but at least he did sweat. Avoid riding during the high heat of the day...get rides in before 10 am and after 6 pm...obviously don't overdo it and be aware of the horse's status. If the horse is blowing very hard and isn't easing, dismount and get him cooled down...slowly. Warm water rinse, start at the legs then work the water up to shoulders and back. Never use cold water on a non-sweater if they are really hot..or any horse for that matter.