My boyfriend always leaves the tap on while brushing his teeth

I'm really annoyed about this. And I have been for months. I have told him over and over again to turn it off while brushing but he always forgets. I also told him that it's a waste of money and water. I don't want to be a control freak but this is so hard to tolerate. I brush my teeth first and I make sure to turn it off while brushing. It's not that hard to do it.