Is a protruding belly button when he sticks his belly out considered a hernia?

Sure Sounds Like One: What you are describing sounds like an umbilical hernia. If small and w/o symptoms, these may be rx'd conservatively. Otherwise, simple outpatient surgery can fix the problem. I recommend that you see a hernia surgeon.

Yes: A hernia near the belly button causes a lump in the area that comes and goes, worse when standing or pushing out with the abdominal muscles. Usually goes away or at least a lot smaller when lying down. The treatment is surgical repair if it is causing symptoms. If the hernia becomes stuck out and won't go back inside it is an emergency and you should see a doctor right away.