kinda long post but I need advice, babysitting nephews and one keeps peeing on himself...

Des

So I just started watching my 2 nephews the other day, the one is fully potty trained they told me so he can go by himself.. However everyday while they are here, he says he'll have to go to the bathroom. And go in there, but minutes after he does he will walk into the room and he has pee all down his pants. Me and my boyfriend have both tried talking to him about it, but nothing we say is helping. I mentioned to my brother that he has been doing it and him and the mother both know,, I assume they talked to him at home and they've said something to him about not going in his pants while he's here when they drop him off. Today he's already done it twice, when usually it's only once a day. The second time as soon as I got his shorts clean he put them on and walked out of the bathroom with them wet. He's now wearing a pair of my step sons shorts because all the clothes of his we have here are drying from me washing them...... But I really don't know how to go about this situation.. I plan on talking to my brother again when he picks them up tonight but does anyone have any tips or information on what I can do to stop this. I really can't have him peeing himself constantly because he'll get an infection and his parents will think we aren't taking care of him properly when we are. And I'm just so lost on what to do.. I'm already struggling to potty train my daughter while 8 months pregnant with my son and this is just a lot more stress on top of that and everything else going on too as we've been struggling with a lot lately. Can anyone please help..?