Why does my little brother always stab himself?

Seriously, I don't want anybody laughing at this. But my 7 year old little brother thinks its addictive to repeatably stab his body parts. He then licks the blood the drips from his wounds. I've told him to stop several times but he keeps on repeating. At this rate, i'm afraid that he might loose to much blood. It was only yesterday when I had to mop the carpet. My friends always think that i've murdered someone.