No. While there could be a simple explanation such as a scratch, you should consult your pediatrician. There may be an infection or communication between the belly button and inside the abdominal wall.
No. But it's probably not dangerous either. He may have scratched it a bit. If the skin around it becomes red, hot, tender to the touch, and swollen, then bring him to his doctor immediately. If there's just a drop of blood, clean it with soap and water and then watch to see if anything develops.