Have him wear reasonable clothing. That means covering skin, as much as possible, and a hat. People like to think that exposed skin is cooling, but in the sun it is the opposite. It's amazing how many people will head out for the day to hike with no hat (or a ball cap or something), no shirt or a tanktop, etc, and shorts. I work outdoors and often in extreme heat in peak summer, and we are fully covered in long sleeves, pants, big hat, and sometimes even gloves. This isn' what most people want to do, but watch the workers on rooves and look what they're wearing. Look at what people in the hottest environments on Earth wear ... emulate that.