If there is no pain in the foot, you will not get a reaction from a hoof tester. I would start by cold hosing and stall rest for a few days. Add bute if you have it on hand. If there is no improvement I would get the vet out asap.
Have your vet out.. your horse should flinch when a hoof tester is squeezed on his foot. He should not be flinching in pain but acknowledging that he feels it