My 11yr Old Grandson Has Learned To Hold His Pee

My 11yr old grandson has learned to hold his pee for hours and hours.....we've watched him drinking lots of fluids and still not going altho he has peed in his pants a few times. He brags that he taught himself to suck up like a straw when he has to pee and it stops it....its so bizarre and now is beginning to really concern us. Can he be doing any damage to kidnes or ??? thanks for your help, his mamaw