Accidents

I was wondering if anyone could recommend an app for my son's phone to help him remember to get up and go potty.

He doesn't really have any problem at home getting up and going to the bathroom, just has the typical accidents during the night which are rare. But it seems like every time he goes to his dad's (which I should really say to see his dad for a few minutes then spend the weekend at his Nana's) I get a call or text about he is constipated or having multiple accidents during the day. I know stuff like that happens but as often as it seems to I feel like something is up with it. So I want to put something a bit more fun on his phone than just setting alarms or reminders, maybe something with a point system or sticker rewards.