This just happened to me a couple of days ago, I was so freaked out I called poison control and he told me it was fine that the worst that could happen was maybe some vomiting or diarrhea, I was so relieved but I still called the animal emergency hospital and they told me the same thing, in the future if your dog ever eats anything you think may be toxic, give them a teaspoon of peroxide to induce vomiting never water.
Hydrocortisone is a steroid naturally produced by the adrenal glands. It is a medication used on both people and animals to treat a variety of skin conditions. Because its naturally produced by the body, your dog should be able to handle it. I'm not a vet, but I believe that your dog will be fine and you will have nothing to worry about.
Give him water to drink and then get him to a vet.