Thank you for your query.
This is certainly not umblical hernia. Hernia mean protrusion of the intestines through one of the openings in the abdominal cavity. Had it been umbilical hernia
you would have noticed a bulge which disappears on pressing and protrudes on coughing.
Pain in the navel area can be due to a number of reasons. The most common cause is contractions of the small intestines. If the pain persists it is advisable to get your son examined by his doctor.
I hope I have answered your query to your satisfaction.
Wishing your son all the best.