It's a lesson he should have learned from Plaxico Burress -- but this was worse. A 15-year-old Brooklyn boy shot himself in the penis Sunday after fumbling with a gun that had slid from his waistband, authorities said yesterday. Khamir Grant was then arrested for reckless endangerment and criminal possession of a weapon -- the same charges levied against Burress, who shot himself at a Manhattan nightclub in 2008, law-enforcement sources said.