Horses skin is different than our skin…you can't tell a horse's age by its skin, you won't see bagging wrinkled skin on a starved horse - you'll see skin stretched over bone, and you won't find extra folds of skin on a fit horse that was once grossly obese like you do in people. It isn't the tongue that tells you what you need to know…it's the gums and capillary refill times. If he's not dehydrated, then it could be a natural thickening/toughening of the skin from years of skin problems. But my personal opinion is that he's dehydrated. That said, I am NOT a vet.