I agree that it could be either boredom or a mineral deficiency. I did some googling, and calcium hydroxide- aka lime- has a bitter, alkaline taste, so he could be missing something. It sounds like you've got salt covered, so if it were my horse, I would call Smartpak and see if they have any ideas. They have so many supplements, and they'll probably know more about what you're already feeding- and perhaps missing- than I would.
Hi Becky ! I thought your horse licking the floor because he needs more salt or minerals that he don't have in his food ! You can buy a block of salt and grip on stable of your horse or buy a food complement
I find most horses are not fond of the lick-it contraptions. Put a mineral block in his stall, often they have the small ones the size of a brick that you can buy a little screw on hanger for that have a good balance of minerals in them plus they are very affordable.