Get some Squeaky Clean Critter Shampoo, or shampoo for kittens. Guinea pigs don't need bathing often, and if he's scratching, he likely has mites, fleas, lice or a fungus.
Is he scratching just in one or two spots, or all over? And how hard is he scratching?
How are the flies getting to him? If you're housing him outside, he may wind up with fly strike, where they lay their eggs in open wounds and the maggots hatch in the pig's skin. Guinea pigs really shouldn't be kept outside, particularly in places like Fort Lauderdale this time of year. They do much better with cooler weather, and can die of heat stroke if the temperature goes about 85 degrees for any length of time. You no doubt live in an air conditioned house -- do your pig a favor and be sure he's in there with you.