Don't listen to the ones giving negative and terrifying comments, he's only 16 for God's sake what is wrong with you people? Anyways, it could be a UTI or urinary tract stone so for now drink a lot of water and see if you have any progress. If there's none or if symptoms get worse let your parents/elders know and go visit a doctor. Nothing so serious at such a young age so don't worry it's going to be okay.