I Have A 3.5 Year Old Boy In My Daycare

I have a 3.5 year old boy in my daycare that is peeing on purpose....
He has been fully potty trained since September and doesn't have accidents anywhere else except at daycare. We have started to use a timer to remind him when to go.... he never goes on his own. Today the timer was set to go every 30 minutes and in between the 30 minutes he peed all over himself and the couch he was sitting on. This is not the first time this has happened. What do I do? He only does at daycare and it's driving me nuts