8yr. Old son has a mass on his right side just below his ribs, no pain or other symptoms, what could this be from?

Needs to be checked. could be a osteochondroma of ribs,benign, or there are some rare bone lesions that need to be ruled out.
Needs specifics. you didn't mention an acuity of event, how long it exists, painful or not, other symptoms. Why not to check with your pediatrician who knows your child the best? and run some tests?
Mass . Difficult to say without further evaluatio and description. For example is the mass hard or soft,movable or stationary.The mass couldbe due a liver, kidney, bony, neural or intestinal mass. Xrays and possible blood work are in order.