Horse stopped sweating(left high and dry)

[Jerald Thiessen <thiesj@tdbank.ca>]

I am looking for a way to get a horses to start sweating . We had some
very hot and humid weather and one of the Saddlebred horses that is used
for endurance has stopped sweating . The horses is in good condition but
will not sweat  , therefore he does not cool down. Last night the owner
rode him about 6 miles and he came back panting with an elevated
temperature. We cooled him down with cool water. It is frustrating to
see a horse fit enough to do 50 miles and not able to safely go 6 miles
because of the heat build up due to him not sweating. Is there anything
I am missing or is there a way to kick start his sweat glands. The
weather has been cool and dry for the last week Horse has access to free
choice loose salt , salt blocks, water and pasture as well he gets fed
beet pulp, sweet feed, molasses, corn oil  and hay.