My 6 month old's belly button it's bleeding slightly. is this normal?

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.