Pain in the right lower quadrant is classic for appendicitis but that isn't usually associated with small bumps which I presume you are referring to skin lesions as opposed to hernia or some other mass under the skin. If the former is indeed correct, the rash & pain could be due to zoster or reactivated chickenpox. You'd notice the small bumps becoming fluid filled and line up in a segment that, if you looked in an anatomy book, might correspond to a dermatome since the chickenpox lives in a nerve root and irritates the area which the nerve innervates.
If you're not referring to a skin rash of sorts, then I'd worry about infections, like abscesses or pus pockets. This might be from folliculitis or infected hair follicles, for instance. Black widow spider bites are often associated w/stomach pain so perhaps the small bumps represent insect bites?
Of course the small bumps might be a red herring but we're trained to find the one cause that could explain everything, in this case, the abdominal pain. However, if we discount the small bumps, then depending upon his age, we might consider colon cancer, rare right sided diverticulitis, Meckel's diverticulum, inflammatory bowel disease, etc.
Best to go see your family physician for a more thorough evaluation than I can offer based upon your description. Good luck!
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