My horse is displaying signs of a sore back? Please help!?

If your horse has a sore back, you'll want to run your fingers down his spine and see if he has any reaction. If so, then yes he has a sore back. What I would do is to stop working him for a few days to let him rest, and give him some liniment baths to help soothe the pain. If he is still in a lot of pain after a couple days (with no signs of improving) then you might want to call your vet.