Noticed a lump in baby's groin, does he have a hernia? Do hernias go away on their own?

Exam required. A bulge in the groin area almost always indicates a hernia which will not resolve without treatment. A baby with a suspected hernia should be examined immediately due to complications such as "getting stuck", also called incarceration. A bulge could also be due to a swollen lymph node or possibly a growth.
Hernia. Make sure that the bump is checked by the doctor usually hernias do not go away definetely if the baby is not acting right or you noticed that something get stuck in the bump call the doctor sooner than later.
Not these. Belly button, or umbilical hernias in infants usually close on their own by about age 5. Groin hernias do not close, and are at risk for becoming strangulated, so should generally be repaired by a pediatric surgeon to prevent it from getting worse.
No. Hernias in babies or children are fairly easy to see due to frequency of diaper changes, etc. As children get older and develope modesty they may not report these changes to the parent promptly. A bulge in the groin or labia in a girl is ahernia until proven otherwise and should be evaluated promptly to avoid complications of strangulation of intestine, overy , or testicle.