Yes, get the vet on the phone and tell them exactly what is going on. And in the meantime I would get some table salt and pour some in your hand, if your horse is used to eating out of your hand, he will probably lick it up before he realizes what it is. If not, maybe mix it in with a handful of oats. The salt will make him realize how thirsty he is. The horse really needs to drink!
~Started young horses in Bosca te Ador, unto the two rein the old Spanish spade, brought them along with two hands that were gentle. Some fine reining horses as ever were made~