As many as 1 in 3 children may be born with a small umbilical hernia. But as the baby continues to develop, the muscles close further and the hernia gets smaller and smaller until it settles into a normal sort of tummy button. This process continues for at least the first two years, by which time most umbilical hernias have resolved. You can easily gently push on your son's umbilical hernia to feel the edges of the gap. Nothing has been shown to speed up this process but until it closes the hernia is unlikely to cause any problems anyway.
Some hernias close themselves as late as ten years of age.
Even when a child has a larger hernia it rarely causes problems. Occasionally a loop of bowel gets stuck in the gap, which may cause tummy discomfort. It can usually be gently nudged back inside but if this is not possible and the bowel is trapped then urgent treatment is needed. It's always best to avoid surgery if possible. However, larger hernias which haven't closed on their own by school age may need a small operation to close the gap.