I'm sorry to hear that your dog has a fever. There are many things that can cause an elevated temperature in dogs, but let's first establish that he truly has a fever. The normal temperature range for a dog is 100.0 - 102.5 F. The temperature needs to be taken with a thermometer rectally to be accurate.
External factors can affect a dog's temperature. If it is hot outside, they are riding or sitting in a hot car, or even if they are playing/running a lot, this can all increase the temperature. At extremely high body temperatures (over 103.5 F) we start to worry about the organs and tissues sustaining damage, so it is important to seek veterinary care in order to safely lower the temperature.
Several diseases can cause fever. Viral and bacterial infections will do this, and generally require IV fluid therapy and sometimes antibiotics to reduce the fever and help the pet recover. Some other diseases that will cause fevers - such as rickettsial disease (Lyme, Rocky Mountain Spotted Fever) or immune-mediated diseases - require special testing to diagnose.
Finally, cancers can cause a fever in many pets.
I generally recommend having your pet seen by a veterinarian to determine the cause of the fever and provide appropriate treatment. If this is not possible and your pet has a mild fever, I would advise keeping him quiet and comfortable, offering water and ice cubes, and placing a fan near where he is resting to help cool him. If his fever is greater than 103.5 F, I'm afraid the best thing you can do is take him to the hospital.
Hope that helps.