The previously posted sugestions are good ones, I'm a PA who Treats children with these types of problems. If he has a urinary track infection 99% chance he whould of complained of symptoms. If he's just being lazy that is a challenge all in itself. This needs to be ruled out as a first line cause. Talk with him, see what you can come up with. If he is doing this on purpose let him know it can lead to a very concerning semi-perminent inconsinence. First thing you have got to find out if he's doing this on perpose if so, stop him, punishments are generally a bad pratice in relation to "accidents" but if it's not an accident spankings, time outs, house chores anything is far game.