What does it mean when a baby boy has a small mass on his lower left abdomen?

Need more info. This is impossible to answer without doing an exam on this patient. If there is a visible mass on the outside of the abdomen it would be different than a palpable mass inside the abdomen. In either case, more tests may be needed to determine the diagnosis. A good pediatrician should evaluate/examine the baby as a first step, then could determine what other testing or imaging might be needed.