How To Get Rid Of Blistery Rash On Hands And Back Side?

My spouse has a quick spreading blistery rash. It started with his hands and then his back side started itching. He was driving a semi at the time and by the time he got into our yard, about 10 minutes, it had spread to most of his torso and down his legs. He was showering and I had him use some Granny's Lye Soap. And now he is soaking in a tub with some epsom salts, Do you have any ideas and what should we do next? I do have calamine and steriod cream here. I don't think it's poison Ivy because it's spreading too fast.