My Dogs Belly And Privates Are Purple, Why?

Have your dog examined by a veterinarian.  Normal dog skin should not turn purple.  Dogs can experience bruising like people which can cause a purple appearance to the skin.  Bruising is most often noted on the non-haired areas because nothing is covering it.  Has your dog experienced any kind of trauma?
Spider bites can cause skin to change red, purple, and then often black.  When the skin has reached black it has died and will slough off.  Is there any evidence of redness, infection, or a bite?
Lastly a blood clotting problem can cause small areas of spontaneous bleeds called ecchymosis--these areas have not been traumatized.  Clotting problems often cause bruising as well.  Again the most common areas to see this are the non-haired-belly, ear pinnae, etc.  Check your dog's gums for signs of petechiae (small pinpoint bleeding) this will look like a series of small red or purple dots--if you are seeing this take your dog to the veterinarian immediately.
Bleeding problems can have several causes--immune mediated, tick borne disease, and toxin (especially anti-coagulant rat poisons).  
Good luck.

He might have a infection or a hernia he needs to go see the vet asap. This could be very  dangerous please go to the vet.